Hello!
There are many good preludes from these two collections.. here are a couple of my favorites
Op. 23 No. 2 in Bb - stay away from this piece... it will put you in the hospital.
Op. 23 No. 4 in D - easiest of the set, but beautiful (I think)
Op. 23 No. 5 in g (I know this one is played a lot, but it is harder than it looks)
Op. 23 No. 7 in c - moderate difficulty, fun to play and beautiful.
Op. 32 No. 1 in C - high difficulty
Op. 32 NO. 12 in g# - one of my all time favorite pieces (however, I'm not so sure it's good for a competition... maybe, maybe not).
Hope this helps you.
Good luck