It seems as though it's just a matter of what caliber you want. I own 2 9mm shields one for me and the other for the GF, we both have trijicon HD sights, 4lbs trigger job, and stippling done. They shoot very very well.
I saw the .45 come out and was impressed with the factory stipple they had on there I thought that was awesome and wish the 9mm came that way out of the box, however, having an 8 +1 with hornady critical defense 9mm is wayyy better than having 6 +1 with .45.
It's the on going war between what's better, 9mm or .45. I rented the .45 and it didn't shoot as well as the 9mm, but if you train with it and put a solid 800-1000rds thru it, you'll get used to the extra recoil. In defensive situations a 9mm is gonna work just fine.
As for the thumb safety? Just leave it off if you don't want it, or TRAIN to turn the safety off while drawing. It's a small pistol so it isn't hard.
Get whatever your heart desires, would rather have 9mm then a .45 carry gun any day.