I have two Macbook Pros - both are comparable in hardware. One is a 17" and the other a 15". The 17" has a slightly swifter CPU clock speed, but beyond that the differences are completely negligible.
I tried a setup a while back where I had the 17" hooked up to an external monitor in the middle of my desk with the 15" laptop immediately to the right of it, and was using teleport to control the 15" from my 17". All development, terminal usage, etc. etc. was being done on the 17" and the 15" was primarily used for email / IM / IRC... or anything secondary to what I was working on. I have a MobileMe account so preferences were synced, but otherwise I didn't really use anything else to keep the computers in sync (I use dropbox/git but probably not optimally).
For reasons I can't put my finger on, this setup never felt quite right. A few things that irked me was
- the 15" was way under-utlized and the 17" was overutilized
- having 2 laptops and a 21" monitor all on one desk actually took up lots of desk space and it felt like I had too much to look at.
I reverted back to just using the 17" and the external monitor and keeping the 15" around the house (and using it very sparingly).
For those of you who are using multiple laptops (or just multiple machines for that matter), I'd like to see setups that work for you for when you have 2 or more machines that gives you optimal productivity and why. I'd like to give this one more shot but with a different approach than my previous - which was using the 15" as a machine for secondary things (communication, reading documentation, etc. etc).