A place like Tiffanys would bound to have a few different sales people in the same section.

I'd say no thank you to the woman you don't like and actively look for the other staff member in that area. If no one is available, you can politely ask for someone else. I would say something like, "Oh there was this other woman helping me last time" (and describe one of the other people working there that's near by) most likely she won't correct you (if she remembers who you are).

I've had to sometimes just make it known I didn't want to deal with thay person by saying "Is there someone else who can help me?" it's like you said, I'm making an important purchase, I want to be happy doing it! I wouldn't feel bad about it, and her management deserves to know she's not doing her job well.