My dilemma is I don't know whether I should learn violin or guitar. I learned violin when I was a child for about 3 years then somehow quit and forgot everything. A year ago I started learning the guitar by myself, but haven't made too much progress beyond trying to learn all types of chords. Now I am thinking about taking either guitar or violin class to make some improvements and for the love of music. Which one should I take?
This is too subjective to answer properly. But I'll try. You gave up violin, for whatever reason. When you wanted to re-start music, you didn't return to violin. Instead you went with guitar. That choice was one you made. Thus the answer seems to already be there.
If you're asking from a different viewpoint - such as will I progress faster/better with one or the other - the answer is probably guitar, for many reasons already aired on this and other sites. Of the two, it will probably be agreed that guitar is also easier to learn without a teacher - although, as oft said here, any instrument will be better learnt with a teacher's help. A class presupposes learning with others; again, I'd say guitar has the edge.
There you are then - between us we've decided the answer - guitar wins! And all this without knowledge of what sort of music you would like to play eventually...
I cannot tell you whether violin or guitar is better for you. You need to compare the two and decide for yourself. Below are some things to consider. Note that I'm trying my best not to be biased towards guitar, being a guitar player myself.
Violin is most commonly used in classical and folk music, as well as in movie and video game scores. There are of course many exceptions to this, for example Mahavishnu Orchestra (which featured both electric violin and electric guitar). Guitar is used more in "modern" music, from blues through jazz, pop, rock, funk, metal, reggae etc. to contemporary pop music.
Violin (and other bowed-string-type instruments) is primarily a melodic (single-note) instrument (not counting double stops). This is why you often see many of them together (ensembles, orchestras). On guitar it is most common to play (polyphonic, multi-note) chords. However in many contemporary styles, electric guitar is also used for melodic play, utilizing many of the techniques also used with the violin, as well as some that are probably unique. On top of that, electric guitarists tend to know more about sound processing than most. You will often see them using effects to alter their tone, for example compressors, flangers, distortions, octavers, delays and so on. This possibly makes the electric guitar the most versatile instrument there is, since you can play both chords and melodies expressively with ease. Note that there are also electric violins.
Both guitars and violins come in cheaper forms, but I'm told a good quality violin costs more than the guitar equivalent.
Guitars can have more technical problems. Intonation, string action, truss rod (neck straightness) and so on need to be mechanically set correctly in order to play in tune without unnecessary exertion.
On violin the aforementioned intonation is up to the player, so a certain technique is required before they start to sound "good". Beginner guitarists have different problems, they will have to deal with fret buzz, and finger strength for chords, before they can sound "good".
There is a lot that can be added to this, so do a lot of own research. Watch some performances of both violin and guitar, borrow both instruments (if you don't already have them) and try them out. Whichever one you choose, you will probably be happy, so don't overthink this.