I think the bags have such different looks and functions that it would be smart to keep both. The Manhattan is a bit dressier than the Speedy and the Speedy holds more (IMO!). I'd keep both if I were you.
Also, selling the Speedy will only get you about half of what the Manhattan costs, so it may not be worth it!
sell it after you get the Manhattan if you no longer use it...if you sell it now and regret it you will end up buying one again...I have heard of that happening to people you end up missing their speedy...
I almost got the Manhattan PM but it didn't have enough room for me. I got the Mono Speedy 30 instead. It seems to hold more, so I guess it depends on how much you carry. I do like how the Manhattan PM looks!