Properly speaking, once a word of foreign origin has been adopted into English, you use English rules for plurallizing it. So, "Focuses", not "Foci". ("Octopuses", not "octopodes".) "Foci" is the Latin plural, and you'll see it a lot in math textbooks, so I won't deduct too many points.
Advantages of the GTI compared to the ST:
- slightly better interior materials and construction; much cleaner, tidier interior layout. I kind of like the Focus interior personally, but the GTI has a more Euro-upscale-impress-the-boss feel. Somewhat roomier feel, especially with the smaller console, and knee room if you drive with a "wide stance". (Which those ST Recaros probably want you to do anyway...)
- Easier to find, cheaper street price as long as Ford salesman can take advantage of the ST hype. Depends on your negotiating skills, availability of X-plan, etc.
- Manual seats even in leather. (This one's a bit of a personal preference; power seats seem like a waste of weight and money to me.)
- Mk6 Golf far along in its life cycle, so bugs have been worked out, parts are available, etc.
- Very nice electronically controlled DCT with shifter paddles. ST is only available in manual, which is just as well given all the reports about the Ford DCT. But the VW one is good.
- Some people say the back seat is roomier. I don't see it. I can sit in the back of either depending on the front seat position; I can't sit in either if the front seats are all the way back. Per the numbers, front+back leg room is identical (0.2" more in the GTI.) I think it's just that the ST seats go 2" further back (43" front leg room rather than 41" -- so this might score as an advantage for the ST for tall drivers that need that leg room.)
Advantages of the ST:
- 8" screen with MFT is better for easy reading of the nav. GTI nav exhibited one error even on my test drive, claiming the road I was on wasn't connected with my destination.
- Slightly better handling. The GTI is quite good, but I'll give the edge to the ST. I had a chance to drive them both over the same route, and there were places where the GTI was doing a fine job holding its line, communicating the bumps, and I was having a blast -- where the ST was simply effortless. Both are excellent here.
- more cargo space. ST loses a bit compared to the regular Focus thanks to the full-size spare, but it's still handily more than the GTI.
- More power! No competition here, obviously. Also, the really wide torque band makes passing and low speeds easy in the ST. Zero torque steer from the GTI, but the ST has no more than a little tug. (Intentionally left in, says Ford.)
- Personally, I like the looks of the ST much better. GTI looks good, but very conservative unto boring.
I chose the ST. The GTI is not a bad choice, though.