The roor perc design blows away the PHX, but alas, they are only made in the US and based out of LA/california, but they don't circulate very far from there.

The PHX design has far too much drag for my tastes, and tends to "chug" - even in a 18mm setup. I have both and rarely, if ever, use the PHX type percs. I don't like to work too hard when I am trying to get high. The Roor perc diffuses the smoke (x3) to create more surface area/better heat exchange, which the PHX does not. The PHX will definitely be easier to find, and cheaper though

As far as interchangeable parts, all GonG bongs will interchange stems and bowls as long as you stay the same diameter..