No, imho extra 0.2GHz is not worth $100. Put these $100 in your mattress towards the next upgrade
Both Xcode and Windows/Visual Studio/SQL Server/Matlab are quite snappy on 1.4. Remember, 1.6 will also reduce battery life. 4GB, on the other side, is a must. And 128GB SSD is a must if you're running virtual machines. Portability of 11" is amazing - I carry it in my jacket (Scottevest, just got it, love it!). You can't do this with 13" model. Resolution of 11" is adequate for coding. Overall, it's a great machine for coding on the go. The best thing for coding is not to stop when you're rolling good stuff - it's interruptions that kill the thought process - and 11" is just perfect for that, it's always there for you.