I have an Alltec Advance, I think its brilliant and all the custys are amazed by it. It was hard to get into though as theres 5 screws underneath it and they're hard to line back up when your putting it back together. Mine is only held with four at the mo as I could'nt get the last one lined up.
The tripple vac is actually 1200 psi and would be more suitable for tiles, with the attachment. If you are just looking to do the carpets, the twin vac - Advance 600 is your only man, and they're signiture porty. Probably never need another if you look after it.