1. How is the 13" (which is 1440x900) as a primary dev machine?
2. If you moved from a 15" MBP, do you miss the screen real estate? Was the MBP a 1440x900 one or the higher-res 1680x1050 one?
I moved away from a (dated) 15" Dell laptop to a 13" MBP for my primary work machine about half a year ago, and these are my experiences:
* I miss my dock and 2 monitors, but not as much as I thought I would.
* The real estate is fine for working with Eclipse/YourFavoriteIDE as long as you're willing to remove panels. (But I prefer hooking up an external 24" for major editing sessions.) The major gain is that MacOSX has way more keybindings for swapping between windows and applications than plain GNOME (or Windows) did, which makes it easier to live with less real estate.
* The A4ish form factor makes it easy to find alternative laptop bags. I've been carrying my MB(P)s around in a freebie a friend of mine got for volunteering at a music festival for the last 3-4 years.
* RAM and SSD are more important to performance than CPU, but I do occasionally max out my both cores and have to wait for some job to complete, but it doesn't really bug me (they were always going to be longrunning jobs). I'm more interested in shortrunning operations being instant than getting longrunning jobs to be faster.
One caveat is that I've never owned or used a 15+" MBP for any significant period of time, so I can't really compare against that, just the 15" Dell running Ubuntu and Windows.
The screen size is fine and I didn't notice the difference going from 15" -> 13". I used my 15" later on though and it was kind of surprising, but I don't feel like the 13" is lacking.