Kemp has more value where you can keep either player...if only because he's a hitter...since neither will be kept then this changes things.
You already have a solid outfield. Verlander is a stud pitcher. He will get you Ks, Ws, complete games, shutouts, among other things.
It's not an insulting offer by any means but I say keep Verlander...use your core OFs and play the wire game if/when necessary for depth.
10-team mixed keeper league (15 keepers), H2H - 1 Win R/HR/RBI/SB/AVG/OPS/W/CG/SHO/SV/K/ERA/WHIP/QS
C - empty (will pick up a hot catcher when ready)
1B - E. Hosmer
2B - J. Kipnis
SS - T. Tulowitzki
3B - A. Beltre
OF - M. Trout, A. McCutchen, J. Upton
UTIL - G. Stanton, A. Rizzo, B. Harper, empty slot
PITCHERS: SP - S. Strasburg, A. Wainwright, G. Gonzalez, H. Iwakuma, S. Miller, K. Medlen, H. Ryu, M. Garza, A. Cobb, H. Bailey, R. Porcello, M. Estrada, W. Peralta, S. Gray, E. Ramirez
RP - A. Chapman, A. Reed, J. Benoit
N/A slot: M. Pineda. DL - C. Maybin, M. Moore