I'd go G36 between the two, but I'd rather have a G30. I'm not a big fan of the slimline, it doesn't conceal significantly better than the regular, and I don't find the G30 to be to wide for a good grip.......that's all personal preference of course, to each his own.
To be honest, both are fine weapons. It comes down to personal preference. go to a range where you can rent gun sand try out both. I did this with the M&P, Springfield, and the Glock. I chose the Glock over the other two, although not everyone will make the choice that I made.
I have both the G 21 Gen 4 & Springfield XDM 4.5 .45 And I like both of them. I've been Shooting the XDM much more lately though. I used it to shoot up some old handloads from the '70's, and was surprised I had no failure to feed, failure to fire, or extraction issues. I searched out all the loose rounds that end up in odd places, car glove compartments, the top drawer of my roll away tool box, junk drawers, etc. Some of them pretty old. Shot them up too. So now I'm kind of conducting an impromptu study to see if I can find some brand it won't shoot flawlessly. Over 1,100 rounds so far. It's like a Cannibal, it seems to eat anything. Still checking.