If you can, go for a triple vac. They are all as good as each-other I presume, I use the Alltec Triple Vac 600psi which I really am impressed with. I wouldn't buy another, because I don't think I'll ever need to. They are a solid bit of kit and easy to work on.
Another machine to consider is the Scorpion, people who have them tend to swear by them and the after sales they get. They are quite simple machines which means they are nice and easy to work on and maintain. As long as you look after it, these machines seem to last forever too in the right hands....
I don't have any experience of any other triple vac, so I really couldn't make an informed opinion on them....