I've purchased from both and can tell you to stay away from Rareprops.
They have the terms of the MPA listed on their site but they have no desire to follow them...at least in my situation.
I purchased an item from them nearly a year ago and did not get the COA from them. I've now written to them 4 times AFTER reporting them to Ebay for not following through on the contract we agreed to and still have not gotten a simple COA in the mail.
I've gotten almost nothing but a bad attitude from them from the beginning. When bringing up the COA, they turned it into something else than it was. The automatically assumed I was questioning their reputation when all I wanted was the COA they said would come with it in the auction. If it's one thing I hate it's when I get screwed and the ones doing it try to turn it around to make it seem like I'm the bad guy for even asking.
Thanks for bringing them up cuz' now I can write to them yet again asking for the rest of my auction to be completed almost a whole year later.