I got both ringed and ABN engines this year from OS - their larger engines come ringed and smaller come ABN. There really is not a heck of a lot of difference to the newer pilot, just follow the proper break-in procedure (which for OS is actually the same, but after reading these forums I am not as quick to lean it out). I feel like I notice more a power curve with more power at the low end on the ringed engine, but it's probably my imagination. Definitely not enough to say "I want this one" or "I want that one". Tower Hobbies sells Super Tigre engines in both ringed and ABC in the same size; I would buy whatever is available or least expensive. I guess one advantage to ringed is that you can replace the ring, or the ring and cylinder, and not have to replace the piston, but if you treat your engines right hopefully you'll never get to that point.

I think the general consensus is the engines with ball bearings are definitely preferred to engines with bushings. The least expensive engines come with bushings (OS LA series, and others), and the high end engines are not offered with bushings.