If you are going to carry a .45, might as well start with one. There's no reason to if you like the 9mm. It's a very effective defensive round with proper ammunition, and costs almost half as much to shoot as the .45.
My plans are if i can shoot it pretty good then i gonna load for it. Then get a 45 just for a truck gun and to carry and not shoot it alot unless i want to make some really loud bangs on steel. I think i may end up just buying both the 9 and 45 and see what one i like the best.