Personally, I don't think you can go wrong with either.
I had a Mueller, but I sold my 535 that was under it with it(only way I could get the price I wanted)
It was easy to setup, and I liked the sight.
However, there is a rumor of an FFIII that might be coming out soon.
http://oldgobbler.com/Forum/index.php/topic,15055.0.htmlIt sounds awsome. If it were me, I would wait for the newest generation to come out.