Paris. Better art and better food.
Is the art really that much better, though? Going to an auction preview at Sotheby's or Christie's is often a far more interesting experience than visiting a crowded museum in Paris. Even a simple auction catalog can be more educational than an audio or guided museum tour, and if there is a specialist around with a bit of time on their hands, you can have a deeper and more engaging conversation than you would get at any museum. The museum scene in London (apologies for the BRTD-esque vocabulary) is not exactly terrible, either...
I dunno, I've never done an auction preview and as for what's most educational I guess the best thing I've ever had was a CD-ROM that explained the Barnes collection (which was notoriously difficult to view in person until the foundation basically went broke and needed a means of sustaining itself), or watching that PBS nun explain art is good too, and yes London has it's "scene" too (which I haven't had the time/opportunity to explore), but Paris is the pinnacle even if it means it's crowded.
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