Well... of course there's a difference, but nothing that diminishes it in function... plus the way you use an iPad vs a laptop is different. Any new computer you get will blow the one you have out of the water.
Check them out in the store before you buy. And if you can't settle for the normal MBP (because of the normal res display), you can always choose to upgrade to a Hi-Res display as an option in the 15" regular MBP for $100... so you get a better resolution in the display (not as high as the retina), but without going all of rMBP limitations - and price.
When it comes to technology, it's always wise to buy what you need, and try to avoid superfluous overpriced extras. Technology is the worst kind of investment: and odds are any computer you buy is obsolete and outdated the moment you pay for it. Hi-Res or Retina displays will become the standard in a few years anyway.
I still think a rMBP would be a waste of money in your case (unless you are a pro photographer or graphic designer?). But if it's mere luxury that makes you tick, I'm sure Apple will appreciate your extra money.
cheers!
