If you could pick one or the other for a 10 day trip - which would it be - what would you recommend (cities and activities not to miss)?

This is just as unanswerable as your other thread "Italy or Spain"

W/o knowing something (anything) about you, what you enjoy, when the trip will be, etc??

Both are biiiiig places - which part(s) of them are you interested in. Have you done any research? As it is sort of like asking "US or Canada - which would you choose for a vacation?"

Good point Janis - I love exploring. I will be in Amsterdam for work the week before and my boyfriend will be in England visiting family. Neither of us will be jet lagged so we don't need to worry about that. We don't want to move around too much. We're looking to pick two (max three) cities and enjoying what they have to offer.
I love wine, good food, seafood, relaxing yet discovering something new everyday. I'm not one for a beach vacation but do enjoy lazy days in big cities.
Our first thought was Venice, Florence, Rome but thought Spain would be great too. I guess we're like kids in a candy store - too many options to choose from!
I'd like opinions from people who have had great experiences from people who have been to different towns. I read a post from someone speaking about an opera they'd seen in Venice that they loved and even returned later that week. To me that's the beauty of this forum - learning about hidden treasures someone else has discovered.
Basically, I'm open!

If you like exploring, 3 cities ( with travel in between) 10 days will be too tight unless you pick something like :
Cordoba, Seville and Granada.
For 2 cities Rome and Florence or Rome and Venice. Or Barcelona and Madrid Or...

Flip a coin--they're both great and would fit your criteria for what you like. Browse through guidebooks and this and other websites and see what really appeals to you. Either one will be wonderful.

I agrre with Artlover... Flip a coin! And do a little homework. It won't hurt.