0.20's are terrible, any little wind or a blade of grass will cause them to veer off course. When I first started I said "0.05g is pretty damn small, the difference in ballistics of the BB's should be almost identical" but I couldnt be farther from the truth. Even in a stock gun 0.25's will perform better than 0.20's. I've been using 0.25's for all of this year except my first game and when I tried to use 0.20's for a game it was so incredibly frustrating because the BB's went everywhere. As long as your hopup is set right, both will have the same range, the 0.25's will have a tighter grouping at any range, the only advantage of 0.20's is they will get there faster but such a small difference in travel time is pretty much meaningless at the ranges airsoft is played at.
Some people swear by 0.20's though, never met any but they are around here on ASC. I have about 6000 0.20's kicking around, always bring some out to a game for chronoing purposes but thats about it.