I would prefer to fit the ecoboost 1.6 turbo from the new fiesta st (or focus 1.6 etc), since its lighter and more powerful. I think the prices are starting to come down for them too.
But in the end, performance wise I don't see a massive difference between the 1.6 ecoboost and a turbo 1.7 zetec. Turboing the 1.7 isn't anywhere near as expensive or difficult. I think maybe the biggest two things that give the ecoboost an edge is economy and reliability (especially at the same performance levels, eg. 180-200hp). But if you aren't so bothered about that, then adding more power to the 1.7 may end up being a more fun engine.
I don't know how far the ST150 engine can be tuned, but if you want to get a lot out of it you will have to turbo it, sure you can do NA tuning but then you could spend more on that than getting a turbo, but end up with less power, torque, driveability etc. Just seems like a money pit to me, not that money pits can't be fun, but lets say you destroy a 1.7 engine with a turbo, its not a big expense to replace it, you can enjoy it and not worry too much about it.
Anyway just thought I'd give you more to think about
merry christmas to you too