It's a bugger isn't it, Wordy? I've had the same problem recently as there seem to be lots of comps right now all closing about the same time.
I recently sent what I consider my best stuff to one that I was really interested in, but did keep a couple back for another one as I felt they would have a better chance there, but it's all guesswork.
I think all you can do is either assume it will be OK and send the best to all of 'em, or try and see if you can get a feel that some work would go down better in one place rather than another. One of the ones I kept back for the second comp was about London, and the comp is for London writers. Now I know they don't specifically want a London theme, but I felt it may hit a target better there...?
Also think about which is the more prestigious, will give you the most impact if you win. Or if you're broke like me right now, which has the best prize money!!
I haven't been much help as like you I'm just figuring all this out. But you're not alone in your dilemma!
x
tc