On the Magic, an inside can be either 184 sq. ft without split bath or 214 sq. ft. with split bath. An outside will be 214 sq. ft with split bath.
I've stayed in the smaller inside, and it's fine for DH and I because his stuff doesn't take up as much room as mine, as no split bath is fine as he doesn't need as much bathroom time as me and we're fine with sharing space. With my Mom and I together, I prefer the larger cabin with split bath as we each just have more stuff and the split bath is really handy with 2 women.
Regarding he IGT/OGT, there are some inside and ocean-view locations that I just wouldn't be happy with, so I personally wouldn't book those rates and risk being unhappy with the location. I'll only book inside and ocean-view when I can choose the cabin. For the *GT on the classic ships, I'm only comfortable with VGT as I'd be fine with all those locations.