The whole idea of recording to external (other) drive was because when Protools was developed, hard drives were slow but also limited by the SCSI-1 bus. This meant that in old NuBus based Macs, internal hard drives were limited to 5-10 MB/s.
The original Protools setups included a Disk i/o card which was basically Protools own SCSI card running an external bus that was meant to be used with Protools but was accessible to the Mac OS as long as the Protools drivers were loaded.
Drive access speeds were also important and another reason the practice gained ground. Depending on your session it may or may not be an issue today. If I was charging money (with the clients in the room AND charging by the hour), I'd use an external but for my own work, it's a non issue.
Ahhh the good old days of running Protools 3.x on Mac OS 7.5.x on a Nubus Mac with 16MB of RAM, 250MB internal drive and some "humongous" 1 GB external SCSI drives.
I'd definitely go with Logic, stay away from Protools till you absolutely need it.
Logic has tons more plug-ins and virtual instruments included.
From the things I've seen on Logic stress tests on audio forums, the 2011 2.0 MBP is getting around the same number of "stress" tracks as the 2010 i7 iMac and even more than some of the 2008 and 2009 8-core Mac Pros.
Also these test were not affected by the hard drives regardless of 5400, 7200, or SSD
Test File
http://www.evan.se/logicprobenchmark/Logic_Multicore_Benchmark_Test.html
Discussion
http://www.gearspace.com/board/music-computers/371545-logic-pro-multicore-benchmarktest.html
Enjoy your hotrod.