I have extensive experience with both a drop-in conversion (USGI M261) and a dedicated upper (DPMS). The drop-in conversion is cheaper but the dedicated upper (or better yet, a whole dedicated rifle) is the way to go. Much more accurate, and probably more reliable to boot.
If you get a dedicated .22 upper, it won't be long before you get the idea to get another lower for it and make it a stand-alone rifle. So you might want to just think about getting a whole .22 rifle to start with.