Personally, I think Koskie will be more of the part time player. I have highly touted Bill Hall this year, so my fantasy reputation is on the line.
Koskie has had a credible drop in HRs and AVG in the last two years. And, while I have heard "experts" say that Koskie is better at defense, they actually were pretty much comparable (Hall, 148 games, 16 errors; Koskie, 76 games, 7 errors).
When you throw in the fact that Hall was switching positions and would, therefore, be more susceptible to errors, Bill Hall is the better option for the Brewers.
I believe that Koskie has been picked up as insurance for JJ Hardy and Weeks. JJ Hardy had a great second half, but his first half was abysmal. Weeks was fantastic when he first came up, but he had a substantial dropoff in AVG. His stolen bases were great and he showed some true pop (13 HRs in 360 ABs), but if you're not going to hit in Miller Park (.236 AVG), you aren't ready for the bigs.
The ultimate question is whether Ned Yost and the Brewers management are baseball smart enough to see Hall's importance to the team (and whether Bill Hall will have a great Spring vs. Koskie).