Sorry if this is a stupid question. In this passage I'm notating, the right hand has accentuating chords and the left hand carries the melody. I wonder whether I need to denote that (and if so, whether method A or B is more proper - I have seen both used); or if it speaks for itself that the left hand has the melody and should be played clearly. It goes on for about twenty bars before both hands should be at the same volume again.
I think that it doesn't speak for itself. However obvious it might be to some performers that the left hand has the tune, that is only their guess, without the authority from you, the composer.
So you need some notation. To me, a louder dynamic (for example "ff") just means louder generally, and doesn't imply any particular any particular articulations. Accents imply a forceful articulation. But then, accents on every note mean that the whole line is just louder. In your examples, you have staccato chords in the RH, so you can't get your first example's ff loudness by using the sustain pedal, so you must get it by hitting the keys harder, which produces the effect that the accents indicate. So in this particular case, it might not make a lot of difference. (Do accents imply non legato?)
But I'd prefer ff because your second example makes me check every LH chord to see if it has an accent.
To some extent you don't need to add dynamics to accentuate the melody. If the melodic line is clear and obvious, the perform should bring it out in some way. But to be sure there isn't any confusion add the specific dynamics.
I think this...
...covers the basic idea. Put two different dynamic markings on the two separate staves.
Marking everything accented makes all the notes the same and defeats the purpose of accents.
Accents are normally used like this (Prokofiev's Toccata)...
...I think especially important in the usage is marking accents that aren't implied by the meter.
But, Prokofiev does use accenting on nearly every note in some places...
...but that is understood to be a barrage of strongly played notes then the music goes back to dynamics.
The two examples aren't going to be played the same anyway. In the top one, there is a tune played by l.h., but that same tune is much more emphasised in the bottom one.
Marcato or agogic accents would align the two bass clef parts better. It's funny that we never mark that the r.h. has to be played louder than the l.h. (in most pieces), which I suppose is where the question came from. For simplicity's sake - which is really what writing out music should be about - the top version is clear, although with the appropriate markings, anyone should be able to translate something like the bottom one.
To an experienced musician you do not need to bring attention to the melody as an experienced musician will see this in the sheet music and they can hear the melodic line as well. For a less experienced musician, noting where the melody travels would be helpful and can be done with a ff notation as you did in your first example. The use of accents also works, but will sound slightly different due to the technique used to play an accent vs playing ff throughout measures. All in all, I would leave it alone, the melodic line is clear in this example. Hope that helps!
The second example is poor, because you can't "play an accent on every note". The only way to hear something is an accent is to compare it with something that is not an accent.
Any performer with the smallest amount of musical intelligence should realize that "the left hand is playing the tune" here, so you don't really need any markings at all, but dynamics marks written below the bottom staff or above the top staff only apply to that single staff, not to both.
I never played piano, but as a lifetime choral singer and a one-time trumpet player, I've seen plenty of music in which the melody (both for the choristers and the instruments) was marked a dynamic louder than the harmony. (Including when the melody was given to the sopranos as usual. As far as, "the melody is supposed to be louder" goes, not a whole dynamic louder.)
As well, my interpretation of an accent on every note would be a bit of a sforzando piano, hitting the note hard and pulling back before the next accent. I could see pianist releasing the keys a tiny bit early, allowing the piano to quiet for a fraction of the note to achieve a similar effect.
So I would recommend A as the likely standard, especially if you want a legato fortissimo.
Any competent musician will know the melody is in the left hand part. There's no need to tell them by adding accents or a fortissimo.
I have recently started writing melodically under the left-hand part in experimental contexts where the melody might not obviously look like a melody. Generally speaking though, including in your case, you should trust the performer to see a melody where there is one; and when there is cause to suspect they might not see the melody, you should seriously consider rewriting it.