If you really do intend on carrying the bike on your back the Carryme and the A-bike are you're only reasonable choices (and perhaps the Strida slung over your shoulder): reason being that when wearing something on your back you don't want it to stick out far behind you because the extra leverage will make it feel much heavier.

However, keep in mind that even though you have to bag it you may still be able to roll it: just leave the bag open at the bottom. Don't waste your money on official folding bike bags which are usually designed to fully enclose the bike, just get/make a sack of appropriate size.

Carryme, Tikit, and Strida would all probably be good choices, but I think the Carryme offers the best value for your money in the sense that it is lighter and smaller than all the others and whatever it can't do you can buy another bike with the money you saved. Plus, with a little padding, you can actually carry the Carryme on your back fairly comfortably if you prefer and it's perfect for riding "too long for a walk" distances.