Arroyo is going to be the 5th starter for the time being, and he had a good outing last week. I've seen them both pitch a lot (i'm from Boston), and I like Arroyo better. Kim has a lot of issues with the stress of pitching in Boston (recall when he flipped the bird to the fenway faithful?), while Arroyo seems more secure out there.
Neither of them is incredible (both are FA's in my league), and they play in the power-packed AL East, but you know they'll get a lot of run support.
If you're set on taking either one, go with Arroyo, and if Kim is still a FA when the season starts, you can always grab him and stash him away on the DL while you see how Arroyo pans out.