I have guns in both calibers and some of each are outstanding, and others, not so much.
If you shoot a group, say 5 shots, and all your holes are touching, the .177 group will be smaller, merely because the pellets are of a smaller diameter. It will appear to be more accurate due to a smaller CTC spread, when in actuality it just shoots smaller pellets.
It all boils down to getting a good gun in the caliber you prefer, and feeding it the pellets it prefers.