I'm tempted by the Ninja, but right now i run a JP FX B5 and it hauls ass!
i really don't see how you can make a 5 port engine run much better than this one i've got like 5 gallons in it and there's practicaly no wear at all.
A friend of mine got a sirio evo 2 and the crankshaft had a lot of play right out of the box an his carb gave him all kinda problems...
i would say get the Plus 21-5 it's the newest engine and i've hered nothing but good things about it. the ninjas aren't really available yet so if you're in a rush you gotta cross that one off.