Test drive, as prev. posted some Glocks in .40. Keep the Walther. Find a deal on a slightly used model "G" gun, get a barrel for 9, (cheaper to shoot),....
Keep both....
Two guns of the same caliber, are better than one, when the Zombies come,...

OBTW, the M+P .40 has a better grip. Even though the Gen4's are a much needed improvement. Prices are compatible, and as others posted.
sellgun=bad....