If I had a higher powered machine, 6ply 20"s are they way to go for trails. Perfect medium.
But I can get around just fine with 18"s on my little blaster, I love the lower seat height and the less rotational mass really helps keeps the wheels spinning and keep it pinned.
I have a few problems, one gearing. even with 14/40 sprockets 18"s just don't have enough top-end and I feel gearing it any higher (sprocket wise) would make it a turd (my bikes pretty stock)
Two tire life.
Three winter time.
four the occasional trail that I just don't have enough clearance for. But usually I can just pin it and go over, I only have a problem if I'm going slow lol.
I do have some 22x12x10 swappers if I need them, but I never do.
Riding with bigass tires feels like I'm riding a tractor and I hate that.
Plus smaller tires look so much better (but thats pretty much my opinion and doesn't mean a thing)
Ultimately if your unsure get 20"s, those are a great medium for everything.