My thoughts: if she wants to suck her thumb, she will, regardless of whether you give her a dummy or not. I reckon dummy sucking is a much worse habit - they can be mighty hard to get rid of. And they're ugly As they get older, kids will naturally come away from teh breast and take their hands out of their mouths to interact with toys and other people. They can, however, still do all that stuff with dummies in, which I think is why they can be harder to get rid of. There's also a tendency to always pop the dummy in whenever kids make the slightest noise, whereas they probably don't always need it, you know?
Having said all that.... I tried to get my kids to take a dummy when they were little as I was so over all the sucking on me. Neither would take it. Neither is a thumb sucker, though they both sucked on their hands when they were young babies.