Help to select best system for usage.

I am planning to buy a new MacBookPro because my MacAir (Late 2010), cannot support parallel Windows OS. The reason for using parallels is because I need Windows Office setup for some of the excel tools and .xml things I work on. I have two options.

1. MD101 (i5, 2.5GHz, 4GB, 500GB/HD, Graphics 4000) and upgrade it to 8 GB or even 16 GB RAM. I can manage the purchase with 8GB upgrade in 1000 USD.
2. I can go for 13-inch Mac Pro (Core i5, 2.7 GHz, 8GB, 256GB, Iris Graphics 6100), which would cost me about 1300 USD.

Please suggest if there will be considerable difference in performance of the two configurations considering my main usage to be parallel OS - http://www.parallels.com/cross-platform-solutions/