Yeah, its all very personal and varies significantly from individual to individual. I, for example, find it significantly easier to write 8 part divisi choir music (or double choir) than solo piano. I can write out a full length, fully orchestral movie score off the rough draft of a screen play, but find it quite difficult just to write a simple symphony.
It all depends on YOU. What YOU find interesting, what YOU hear in your head and what comes naturally to YOU. My best advice would be to experiment a little. You should be able to at least get a general idea of what area fits you best pretty quickly. Try writing a string quartet, a piano solo, a vocal solo w/ accompaniment, an SATB choir piece, a symphony AND a few different instrumental solos (plus whatever else you can think of like concerti, advent garde, electronica etc.). You don't need to finish each piece, but you should push yourself until you have written at least 1:00 of play time on each - sometimes you feel like it's not going anywhere, but you just need to hit your second wind. See what comes naturally and easiest and go from there. Focus on that for a while and then try your hand at incorporating the experience you've picked up into a new area.