I carry a 642 partly due to convenience and partly due to its attributes as a carry gun.
The 642 is light and compact, but I can also shoot it reasonably well. That means I'm much more likely to actually have it on me if I were to need it (except, of course, where prohibited).
I do tend to shoot DA revolvers better than most other gun types. I also like the fact that I can do dry fire drills that mimic everything in live fire except for the blast/flash/recoil. With a semi-auto I can't do dry fire drills that result in slide lock unless I manually pull the slide back. I can't practice DA/SA transitions without cocking the hammer after the first shot.
Another advantage to the revolver is that I don't have to worry about the slide being pushed out of battery if I need to make a contact shot in self defense.
Some people will say that revolvers are more reliable than semi-autos, but I think that if we're talking about good quality, well-maintained guns any difference in reliability is going to be insignificant.
Having said all that, I'm definitely not opposed to carrying a semi-auto. A Sig P239 is at the top of my list for semi-auto carry guns that I want, though the Beretta PX4 Storm Compact Carry is climbing up that list. I also certainly wouldn't mind carrying a Glock 19. I have a Beretta 92FS for home defense, but the main reason I don't carry it is because I don't have any decent holsters/mag pouches for it. Yes, it's large, but carrying it concealed is certainly do-able with the right gear and clothing. And I'll admit I would prefer to do a G conversion (decock only) before carrying it.
So, in my long-winded fashion, what I'm saying is that it's all up to you. There's certainly nothing wrong with carrying what you know and what you're comfortable with. It may even be preferable. But there's nothing wrong with wanting to branch out, either.
I'd suggest, if at all possible, going to a rental range or borrowing a .45 Shield from someone and giving it a try before making up your mind. You may find that it suits you well or you may find it doesn't. But you'll have some experience on which to base your decision.
Just my opinion.