I know you can read minor scales in both sharp and flat keys, but which is preferred in the world of music?
Is it more useful to know them best as say Eb minor or D# minor?
I know it doesn't make a change in the scale whichever way you read it, and as long as you know the relationships of sharps and flats you can figure out which is which in the opposite key type, but I just want to know which way I should learn them.
The way the circle of 5ths is setup, you would typically use F# minor instead of Gb minor, C# minor instead of Db minor, G# minor instead of Ab minor, and Bb minor over A# minor. You can uses either either Eb minor or D# minor because of where it falls on the circle.
So 3 keys you would use sharps for, 1 you would use flats for, and one is rather interchangeable. It makes sense that sharp keys are slightly more likely then flat keys because the difference between major and minor is 3 lowered notes. See the picture below:
To make the scales more friendly to newer musicians, you typically want to frame a scale in such a way that it seems to have the notes modified as little as possible. To contrast the Eb and D# natural minor scales:
1 Eb D#
2 F E# -> (F) (we usually consider E# as F)
3 Gb F#
4 Ab G#
5 Bb A#
6 C B# -> (C) (we usually consider B# as C)
7 D C## -> (D) (we usually consider C## as D)
8 Eb D#
As you can see, more work must be done to translate the D# minor scale to the actual note, with a double sharp (in the melodic minor), meaning the note shown on the scale is C, but the musician must modify it with two sharps, and the actual note played is D.
In general, it is preferred to use the Eb minor representation for the above reason. However, if you change keys in the piece, perhaps to a more accessible sharp key, it would be reasonable to avoid switching from flats to sharps, and to stick with the D# minor representation.
If you don't have a particular piece in mind, start learning the Eb. But then work your way through all of the keys so that you can more easily handle more difficult pieces when presented with them.
The simple answer is that you use whichever one has less sharps or flats. So, five flats over seven sharps, five sharps over seven flats, and either six flats or six sharps (they are both the same). This is just as you would do with major keys.
Perhaps the reason you ask the question about minor keys is that a minor key always has either three less sharps or three more flats than its enharmonic (i.e., same name) major key. So, C major has no sharps or flats, and C minor has three flats. B major has five sharps, B minor has two. Where it gets tricky is here: G major has one sharp, and G minor has two flats. (Get it? Two flats is three more flats than one sharp...sort of.) And here: Db major has five flats, and Db minor has eight. Except there are only seven notes! So we say C# minor instead, and it has four sharps. (C# major, same key as Db major, has seven sharps.)
So, again, we generally go with the simplest way to get the key we want.
Note that this is all only true when you use "equal temperament" as opposed to other forms of tuning. I can't speak for other instruments (and for other instruments I'm sure what I'm about to say is less accurate), but if you're working with the piano, you can safely ignore any other form of tuning for twenty years or so without any negative impact on your musical development.
It's not just minor scales. We also have the choice between C# major and Db major etc. As a rule of thumb, pick the one with the smaller key signature. Eb major has three flats, D# major has nine (double sharps count as two). You couldn't even WRITE that key signature! So no contest. But don't be pedantic. Db major has five flats, C# major has 7 sharps. But that isn't necessarily a shoo-in for Db major.
There are actually two aspects to consider in order to answer to this question. One is about the practical aspects related to music which is intended to be played on modern instruments with a basically european heritage. In this regard, there really are two ways to describe one scale (although each way still might lead to different chords being used as harmonies) and one is probably more common. This aspect has been addressed in other answers.
To address the second aspect, which is that Eb and D# actually are not different names for the same scale, one has to dig a little deeper. When starting from basic physics, there are several ways to tune an instrument "correctly". For a long time many different tuning methods, e. g. pythagorean tuning were used. In those systems Eb and D# were actually different tones and, following from this, Eb minor and D# minor were two distinct scales. This only changed in the Baroque, where the invention of the well temperaments first allowed for instruments playing in all keys reasonably well without retuning (and where, as a prerequisite for this tones like Eb and D# were the same frequency [1]) and then the equal temperament [2] which is the way most modern instruments are tuned.
Incidentally, most classical or jazz musicians I know still make a difference between e. g. G# and Ab when talking about music, even if (at least on a piano), they refer to the same tone.
[1] see wikipedia for "enharmonic" [2] see wikipadia
There are two primary considerations when picking a key: what are you most comfortable writing in, and, more importantly, what is most comfortable for the instrumentalists who will be performing your piece?
I recommend you read the top two answers to this question: What's the point of keys others than C and Am? Briefly, which key you pick well slightly affect the sounds that your musicians will be able to produce and how easily they'll be able to read your music. For example, string instruments have a fuller and brighter sound in the keys of A, D, and G major and their relative minors because they get to use strings in open positions.
More importantly to your question, string players most frequently see music written in keys with sharps instead of flats. So if you want to write a piece for strings in the relative minor of A major, you have to choose whether to call it F# minor or Gb minor. Strings would be more comfortable reading F# minor. In contrast, jazz band and symphonic wind ensembles members usually see keys in flat key signatures and might be more comfortable with Gb minor.
And finally, what key you use is somewhat of an arbitrary choice. If you can't decide whether to use a flat or sharp key signature based on the instruments that will be performing your song, you should pick based on whatever key you personally want to write in.
In other words: what key you pick is a matter of arbitrary convention, if you are dealing with equal temperament. But it might be a practical matter of ease of sightreading, say for saxophonists or brass who are more used to flat keys.
