Not sure which rMBP you're comparing...but the 15" model has more 'ports' (I/O)---including a second Thunderbolt and HDMI along with an SD slot. The SSDs are spendy...but nice n thin, a different form factor than the typical 2.5" SSD, which allows for a smaller more compact body of the computer itself. While the $400 'up-charge' for the 768 (vs. the 512) seems steep, these are proprietary drives...and typically a decent 256GB SSD goes for 225-250 (Sam 840) with a few bucks shipping...as well as time invested putting it in. If you're savvy and can do the upgrade yourself with confidence, obviously this isn't relevant (pending what your time is worth)....but if you have a computer shop/repair joint put it in there for ya, probably another $50. Taking a look around New Egg for SSDs larger than 512GB, prices skyrocket. You can get an out of stock Intel 320 600GB @ $850....an OCZ refurb 1TB @ $1500...or the Intel 910 PCI-E 800GB for a buck under $4,000! $400 seems pretty reasonable to me for the upgrade...the Apple Tax, as buyers and lovers of Apple gear, we all pay it. In this case, an extra C Note for the upgrade and everything that blade SSD allows the enclosure and computer itself to be...I'm not sure I would call the upgrade 'obscenely expensive'.
The rMBP is just over 4 pounds (15"). It's not a large computer...bigger than the 13, yep! But the power, speed, display and GPU are all 'better' on the 15", especially right now...as we wait on the iGPU Haswell update to the 13". When that happens...it may be a different story.
Good Luck---You can't go wrong, regardless

You're looking at the 'right' computers IMO. The Retina models are extremely impressive---but you may benefit from a cMBP. If you've got a local Apple Store, it's worth the look. Hard to explain but the feel and body of the rMBP (both 13 and 15" models) is incredible!
J