I have a 9 in Service and Tactical. I have a close friend with a 40 in Service and Tactical. We BOTH like the Service guns better than the Tacticals.
I also had a Tactical in 40 and now have a Service in 40. Both shot about equally well, but I think the Tactical 40 hit closer to the sights than my Service 40 (a bit high). My Service 9 is MUCH MORE ACCURATE than my Tactical 9 with the stock barrel. I now have a BarSto in the Tactical 9, and it now shoots a little better than the Service 9, but the accuracy difference between the two now wouldn't matter for practical (defensive) purposes.
The accuracy of my Tactical 9 with the stock barrel was very poor grouping around 4" at 50'. The BarSto cut that down to well under 2" at the same distance for 10 shot groups from a rest.
My Tactical 9 had machine chatter marks in the factory barrel that I believe were the cause of the poor accuracy. I've seen other new XDs at the store with this same problem so now I know to look for that and suggest others do so as well.
My opinion is that unless you need the extra barrel length for extra velocity to make a power factor, you would be better off with a Service. The Tactical has one other (at least theoretical) advantage in that weapons mounted lights or lasers would be farther from the muzzle, and would be subjected to less blast. FWIW.