I have a 15" MBP, 13" MBP, an iPad 64gb 3G, plus I have access to my old MBA if I wanted, my wife uses it on occasion now. I've thought a lot about changing/improving my set up over the last while, here's my experience and suggestions.
The 13" would work if you planned on hooking up to an external monitor often, that's provided you do a lot of your work at a desk and can plug into the display.
If you're doing most of your work away from a desk/large display then stick with the 15", you'll appreciate and be more productive with the larger screen and it's only 1 lb heavier and 1.5" wider than the 13".
Working on a 13" screen is okay, for a few hours a day. But, if you're working / reading from a laptop display many hours everyday then the 15" will be more pleasing to use.
On the other hand, aside from screen size being a little cramped at times the 13" MBP is a GREAT laptop. You will like it more than your x301 - I sure did. The 13" seems like the perfect size and weight, it's the notebook I always "want" to bring on my travels, then I think about what notebook I should be bringing and because of the screen size alone I try to bring the 15" if I can. For example on a commuter plane I'd never bring my 15", seats too small.
I think the ultimate is to work from one laptop / computer only, for that the 15" is the best choice. Many people because of their needs or the geek in them want another smaller laptop for travel, couch, cafe surfing and right now that's a tough call on which way to go.
I'd consider the iPad as something for fun and media consumption, I'm writing this post on my iPad now, it's fun sure but about half as fast as typing on my notebooks.
Without an external display I wouldn't go to a 13" screen full time, I've tried it often only to begrudgingly go back to the 15" ... never happy about the size and weight but very happy when I open it up and start working from the larger screen.
HTH