Question about petticoats and underskirts

Hello~! I'm quite new to lolita and i'm about to make my first purchase. As I've been wearing lolita-lish clothes since I turned 13, 

I think the most important thing I need for now is a decent petticoat. I have heard that Dear Celine has some nice ones. 

My question is, which one to choose? I'm (as I'm over 20-year-old student of medicine) going for the "classical" look; elegant and sophisticated. I'm not sure which petticoat will suit me, though. I'm thinking of an A-shaped one.

Is it better for me to get a medium-puff A-shaped, bell-shaped or an underskirt?
What is the difference between an underskirt and petticoat? Will underskirt make a dress look puffy?

I want to buy this dress (http://lolibrary.org/node/14326) (Chocolate Rosette in Navy) and I'm not sure which pettitcoat will look good with it.


Thank you for your answers~!