Tavarez and Seanez are both pretty good pitchers, but the numbers seem to lean toward Seanez.
Seanez in 60 IP last year had an ERA of 2.69, 84 K's with just 22 Walks, also only one loss credited to him with 4 HR's given up all year.
Tavarez in 65 IP had an ERA of 3.43, with only 47 K's, and 19 walks, three losses credited to him he aso gave up 6 HR's.
What you want in a middle relief set up guy is a high K count, with as little home runs as possible, so the scale tends to tip toward Seanez. But given the constant injury of Foulke I think it would be safe to keep them both.