First, please let us know what apps you will be using most. Based on your description, a MacBook will be fine for you. It will be multiple times faster than your current machine. Also, OS X is a much snappier OS, which makes the computer feel faster. As houltmac mentioned, spaces could help you overcome the 13" screen. I'd take all the money you save and go get an external hard drive for backups, an external monitor, and any other accessories you want. You should read
this thread for tips on purchasing a new mac.