The decision is really dependent on one factor - What you want from your car. The zetec has a very lazy power delivery compared to a tuned crossflow. Don't get me wrong, it's very fast in 165 spec only and you can tootle along in silly gears say 5th at about 10mph if you want. But you need to ask yourself if that's what you want from your sportscar. I'm actually really pleased with mine now. I was a little reticent once the engine was installed from new but I now have about 2K miles on it and it's loosening up.
That was point one, the 2nd point is that there is no noise like an XF on song and you don't get that same noise with the zetec, personally I miss it, but not on the long touring journeys to Scotland or on any MWay.
I think fuel consumption is poor in the zetec although around town it seems to be very good (you don't get that power for free).
For reference I used to have a 1700SS with a head by Roger. The head cost about £700 and was worth every penny, it transformed the car even on the standard bottom end and cam. The engine seemed to want more revs than my 234 cam could allow it though which makes sense I suppose.
In the end you can see which choice I made.
Nigel Mills - 2.0 Zetec carbs