You should always keep Kotsay IMO. Finley could work in Safeco, but I had him be mediocre in Coors. Below are some Kotsay stats.
Yankee-191 29 11 6 20 12 2 1 .335 .487 .381
Petco- 419 46 25 6 49 31 2 2 .325 .432 .372
Fenway-629 105 30 17 92 64 3 5 .337 .475 .393
Kotsay obviously won't always be that good, but as you can see he's definitely capable of putting up great numbers.