Its really hard to say whats better.Ive been riding for 6 months (5 of it was on mod) now ive moved to stock iam a bit shorter than you somethins are easier some are harder.
If there is any possibility try some stock from friend...
Have you ever ridden a regular mountain bike... stock is same height etc. but the geometry of it is much better....
My opinion about frames is this... stock=longer version of mod on bigger wheels.
Btw these riders ride stock the best (in my opinion also there are some other good guys) Kenny Belaey,Vin Hemance,Gilles Coustellier,Neil Tunnicliffe or Damon Watson.
Iam still new to trials so i dont know many pro riders but only i know on mod is Benito Ros.
(buy 24" if u cant decide :DDD)
Dušan