Hey bro,
Ive been working alot with the 64 bit mode of Logic, with the exact model.
Firstly, the most important thing is the Processor. If you wish to get the 13 inch, get the i7 because it has about double the performance. It will have 4 virtual cores instead of only 2.
ALso, 8 gigs of RAM is a MUST!!.... Ive used it with 8 gigs on my 13 inch with the older core 2 duo. You should know that some of my production uses a shyt load of effects and sounds aswell. WHen i ran it in 64 bit mode with 64 bit vst's, it was a beast. I loaded many tracks of Stylus, Omnisphere, and Trillian EXTREMELY easily. I loaded a FULL rack of 8 basses as a test of the RAM. It was amazing.
btw all the vst's u use are in 64 bit now like Battery, all Spectrasonics, Kontakt sampler etc etc...
Lastly, the harddrive.. I have an SSD on both my machines which is the iMac i7 Quad.
If you noticed, the HD meter in logic isnt really that high even on a slow 5400 HD.
If you want to save money, just get the 7200 speed HD. It will be fine.
The SSD is great for loading things super fast, but it is not needed.
I've been using the SSD for a year and its great, but the 7200 rpm HD is good enough for the job.
Irie Vybes.
For more info on testing the processor check out the EVAN's test... This is very popular where we use this standard test to rate the performances of Logic on different specs.
http://www.evan.se/logicprobenchmark/LogicMulticore_BenchmarkTest.html
Recommendation.
Get the Macbook Pro 13 inch i7 with 8 gigs of Ram, with a 7200 rpm hardrive.
with 64 bit Logic and supporting VST's.
http://www.gearslutz.com/board/music-computers/371545-logic-pro-multicore-benchm arktest.html