New rMBP vs. MacBook (Lap Use)

I currently have a 2013 MBA and it works pretty well for me. I do a lot of programming and enjoy watching 1080p Youtube videos when I come across them, and the MBA has never given any issues for both of those.
I'm interested in upgrading, mostly for the screen. My options are obviously the MacBook and the new 13" rMBP. However, I do have an important requirement.

The vast majority of my laptop use is spent with the laptop on my lap, sometimes for upwards of 8 hours straight. The MBA is perfect for this: very comfortable, small, extremely light, and very little heat emission. I hardly even notice it's there.

I don't care that much about the new MacBook's flashy features and form, however, I absolutely need something that's comfortable to use on my lap for extended periods, which means not too much heat (regardless of what clothes I'm wearing) and not too much weight. The rMBP is only about 0.5 lbs heavier, so I'm not terribly concerned about the weight, mostly just the heat and the dimensions.

I know the new rMBP was only just released, but I'm wondering if anyone with the second most recent model could tell me if there's a good reason for why I should prefer the lighter model for extended lap use, or if the MBP is perfectly fine.

Thanks.