It's pretty much personal preference. I used to ride stock (26" wheels), but moved to mod (20") a couple of years ago after my bike was stolen and a friend lent me his spare bike. My stock turned up around 4 months later, but by that time I was really enjoying riding 20" so have stuck with it.
Generally stocks are better for power moves, while mods are a bit more maneuverable so better for techy riding.
I have to admit though, after climbing more and more over winter I'm seriously finding it hard getting back on the bike. My motivation is very much still with climbing right now