If you are leaning more toward the 9mm Shield, then get it. BUT, if you first get only the chance to get the .40, grab it! As has been said, you can always convert the 40 later if you still want to.

I really like my .40 Shield, and haven't had any problems getting almost instantly back on target for quick follow-up shots with my 180gr range ammo. It also points so well for me, I can practically ignore the sights, and I'm getting short-string groups as tight or tighter than with my with my FS M&P9/40 at the same ranges. That impressed the h**k out of me!