I'm sorry if this has been asked before, but I've googled all over and can't find the answer -- I've heard raving reviews about Vivi Novas but when I went to buy one, I find all these different types, V2, V2.5, V5, V8, V9, V10, etc... but I can't for the life of me tell what's the difference. It looks like they all can come in various resistances and tank sizes, and they all look mostly the same. Is any one better than the others?