There are a few reasons for choosing the MacBook.
1. It's small and lightweight. I have Degenerative Disc Disease (I've had one spinal surgery and it's pretty much a matter of time before I have more) so carrying a heavy or bulky laptop in one hand and a cane in the other isn't an attractive option. It's a long and painful enough drive as it is; I have no desire to make the experience more painful.
2. It's durable and powerful. Most Windows machines either aren't powerful enough to handle high volume OCR and bulk file uploads without burning out after a few months. (I've had especially bad luck with Windows machines and stopped using them after I had three laptops die in a single year.) The smaller Windows machines aren't as durable as their Apple counterparts, which are built to last.
3. It's more compatible. While I will be using the scanner's Windows-only software to process the documents for now, that may change if I find better software. Macs can boot into Windows and OS X (along with Linux and its cousins) while Windows can't handle OS X.