I would encourage you to find websites that are currently using either one. Before investing the time and money into either, research if the storefront that you get will actually meet your needs. Will you be able to display your items in a way that will give a potential buyer of your goods a feeling that s/he is buying from an established seller.
My experience with Channel Advisor's MarketplaceAdvisor storefront was that after spending the effort to setup the store it didn't give me the feeling that a visitor to my store would feel like s/he is doing business with a real outfit. It was too toyish. I decided to take my storefront offline. I think I could have gotten a more serious looking website from them but they wanted several thousand dollars to set one up for me.
As far as your questions. I am pretty sure that either of the two companies would help you get your inventory loaded into their system. But that is the easy part.