Gearing is relatively easy to change. IIRC, the "mid-compact" 52/36 on the Tarmac still uses a 110BCD spider, so you'd only have to buy compact chainrings to convert it to a compact 50/34.
The difference from the 11-32 to the 11-28 is obviously that one bail-out gear. I personally run the 11-32 cassette and do use that bail-out gear, but very seldom. Not a single one of my other riding buddies has a gear that small, but many do have the 11-28 cassette with a compact crank. I think you'll find there are very few of us out there who actually need that 34 front/32 rear gear. I'm a clyde who likes spinning up really steep stuff, and I love being able to get down into that low gear and keep spinning as long as I want, but most people don't need it. Even for me now, I find it nice occasionally on switchbacks, but if I was buying a bike now I wouldn't worry about missing that 11-32, I'd be fine with 11-28.
If you get the 11-28 and end up wanting another gear, then it depends on which bike/brand you have. If it's SRAM, then it's simply getting a new Apex, Rival, or 10-speed mountain bike derailleur and a new cassette. With Shimano, I'm not familiar with the interchangability of their parts as far as how to get another gear there, but I think they do make a longer cage in the 10-speed Tiagra now.
Of course you can always go the other way too: if you get the 11-32 and find you don't need the 32, but want smaller spacing in the rest of the cassette, just swap out the cassette.
I guess what I'm trying to say is pick the bike, then figure out the gearing you want. Many shops will also give you even money on swapping out parts on new bikes too, if you love a bike but want to change the gearing.