Hmm the cost of the i3 is Now: $127.99
And the 6100 is Now: $149.99
Now if we look at the CPU scores of 3DMark vantage the i3 scores 10.156 and the fx scores 14.687
So, if we devide those two numbers 10156/127.99=79.35
For the fx we get 14687/149.99 we get a socre of 98
So the 6100 is a better buy, and AMD mobo are also alot cheaper. So i dont see why i would get a i3 2120 over a FX 6100.
The intel build is the way to go at your price range. As stated (and referenced in links) the I3 will game better than an AMD build and will provide better upgrade path before needing an entire new rig (the LGA1155 platform still has Ivy Bridge to go through). Your original build isn't bad so stick with it if you really want AMD, but consider something like the I3 combos above or the build below. You will also find that Matlab and CAD will be more responsive with the faster Intel architecture.