Not an easy decision. Kim is certainly in the zone now, which makes it hard to deal him. I haven't heard that the job will be Mantei's upon his return - where did you see that? If it's true, then you have to make the deal, no matter what the stats are now.
Even if the D-Backs are undecided about the closer situation, there's certainly the possibility that Kim could lose his job to Mantei, especially if Kim hits a rough streak. Given that danger, I'd take Foulke. Job security is the most important thing in a closer, and Foulke should improve his overall numbers.