X-flow sounds better, has (much) less power, worse mileage. Depends on use of car. If auto-x/track days, extra power is nice. If blats, who cares? For blats I preferred my X-flow due to the glorious sound. Naturally, it's nice having 2x the power at the track (tho harder to actually stay on the track!).
Very difficult, not impossible to swap X-flow out for Zetec, even harder to swap for Duratec (intake/exhaust are opposite). Happy hunting. And remember, there is no bad se7en, just varying degrees of goodness:).