You can certainly use the internal PB display along side a 23" display. This would give you both screens to do with as you please. You don't have to switch since they're both on all the time. Just drag whatever you want to whichever screen you want to use it on. Most apps will remember which screen they were last used on even if you shut down your machine.

As for which device? I think most would tell you to get the Apple display, but the Dell is no slouch and should work well. FWIW, I think the Apple is a bit better quality for sharpness and color but you might not have a need for that fine a line in order to save quite a few hundred bucks.