I am also facing the same problem... but m more towards the k5, cause:

1) The design is great, no doubt about that...
2) I am 185cm, when I sit in the focus, i need to push the driver's seat almost all the way back, then left with a space only enough for kid..
3) The optima has been lauched for quite a long time now in US, so far, no bad comments, or and recalls.. for tat u can always check youtube and surf around the net and look for it...
4) Ok, may be this is a bit too much, but the glove box is kinda too small in focus, and I dont like the centre arm rest cabinet, cause if u see carefully, it is held closed by plastics, I mean, both hook and lock are made of plastic... (feels a bit cheap)
5) The push button start for focus, is not really a nice 'push button start' system, wat i mean is, you need to press and hold on to the button till the engine stays on, whereas most other push button start system, you just need to press and release..
6) Although focus comes with push button start, but you can notice they didnt redesign the dash properly, cause it wont take long for u to notice that there is actually a hole (looks like originally for the key ignition) which is covered by a plastic cap.
7) There is no doubt the driving experience is better for focus, you can really feel it, just the steering and suspension, ford has done very well here...
8) The DRL on focus is quite dim, or may be the angle is directed towards the floor, cause I can hardly make out the DRL in focus, even in the night...
9) The parking assist is awesome, although I hav only tried it in a wide parking space, since I couldnt find a space thats narrow enough... but the focus parks the car nicely in the spot... but also, I hav asked the SA regarding the parking assist, he honestly told me most drivers used the parking assist is only 20% of the time, cause it wont be able to detect parking space thats too narrow (esp in m'sia) and which comes to my main point, the front and rear parking sensors do come in handy
10) this is my personal preference, I dont like the handbrake design in focus, it feels like the break is pulled out of the central console next to the gear shift, worst design ever ( for me)

Those above few points, are the few things I observed when I test drove both cars, these are my views..

Gud luck