I would have to agree that if you already like the shield, the one in .45 sounds like a good option. It is also one of the most affordable.
Having shot both the XDS and the Shield 45 side by side(I own the XDS, brother owns the Shield) they are very similar, both amazing and totally depends on your preferences. The Shield feels very nice in the hand and has a "better" trigger than the XDS. If Springfield incorporated their Mod2 gripzone to the XDS, I would be all over it. I am not a big fan of the feel of the grenade style grip in the hand, although it grips fine. The XDS has a heavier trigger, but I actually like how it breaks after you get it broken in. However, I strongly prefer my XDS with a 4" barrel. Helps tame a little more of the recoil, which isn't near as bad as I was expecting and gives a longer sight radius. I actually shoot this gun far better than I expected as well. My accuracy is on par with full sized semi auto's.
If I were to go back and do it again now that the shield 45 is available, I would still choose the XDS in the 4" barrel. If I was limited between the two having a 3.3" barrel I would choose the shield. Probably doesn't help you too much. They are both great but different so it's really up to you... among any other options you are considering.