I concur 100%. I started shooting IPSC with a Glock 22, and then moved on to the G35. I could shoot both well, and liked the simplicity and reliability of the Glock.
Wanting to see if an S_I could improve my game I switch to an STI Edge and really, really liked the feel of the more solid steel frame.
After shooting Limited with the STI for a while, I decided to try the Glock again in Production, and it just never felt right again. I found it flex'd a lot more during recoil and was a lot more difficult to get back on target.
If it were me, I would stay with a steel frame if you plan on making your ST your main firearm. Beyond that, get out and try as many as you can and see what feels right for you. I find choosing a right firearm is as much about ergonomics, what feels right, as anything else.
And as for ammo, for simplicity sake I would stick with the .40 rather than introducing 9mm. Just easier, my 2 cents anyways.
Glad to see you getting involved in this awesome sport, enjoy!