With the tactical you will get a little more velocity out of your bullet and bit more accuracy.
And the service is just an overall great pistol. It handles well, accurate, trigger pull is almost like having a single action, and fast. Where you point the pistol is pretty much where you are going to hit.
All and all whether you get the service or tactical you will have a great pistol.
I have the 45acp and 40sw in service and never had any problems no matter what ammo I've used.