That's kind of a tough one.
I ech Cyantist, in that if you have double din space, look at that more closely as it will be a cleaner install overall. Also, the w200 utilizes Alpines new "full speed" iPod connection which is way nicer and quicker than the old Aux way that I have and the 310 offers as well.
However, the w200 does not have time correction, eq settings, x-over, and other settings the D310 has. You can buy the H701 Processor (which is what nearly everyone I know has done) for the w200, but that's another $400-500, and since your friend is at BB, he (as far as I know) can't get the processor for cost since they won't carry it in the store.
Also, IMO the flip-out 7" screens look a lot nicer than the w200 6.5" screen.
Bottom line, if you'd have to fab double-din space, DEFINATELY go with the 310. If not, then it's kind of a toss-up, b/c the w200 will look more factory, but I'd still go with the 310 because of the functionality and extra .5" screen size.
EDIT: I just realized it's going in a Civic. I just bought an 06 LX Saturday, and will be putting in my D300 next week.
If I were in your case having the choice of a brand-new unit, I'd look at
the w200 really hard.