I've had a Powerflow in the past (on a Civic) and it was fine. Not as good as the Greddy system I replaced it with but good for the price (mine was £300). However, the exhaust depends on the fitter as many have said.
As for size though, I'd go for a 2.5 inch. Don't forget to make sure they mandrel bend the system as well (to make sure it's the same width all the way through).