I personally like the cpr dual bak pak. It seems to work really good. I know that a lot of people do not like the prizm's, but I have to say, I personally like them. I've used them for the last 4 years, and even put a pro deluxe on my new tank. Some say it's noisy, or that it needs too much finicky work, but my experience is, once it's set up, after about a week, all I do is check the cup and it works exceptionally well.
The cpr though, I REALLY like. You can even find some 'knock off' versions which will do the same thing, just less money because of the lack of the cpr brand.