Hi there, I recently sized this consideration up and with with the 12" MB. Like you, I have a top of the line 15" rMBP through work and a 27" (older) iMac at home.

While you didn't say it I'm assuming you want the 12" for an on-the-go, everyday carry to take between home and work. At least that was what I was seeking. I do a bit of design and development but really got it for research and writing papers (yaaaa grad school).

All in all, it's a fine little machine but I would probably recommend you go with the 13" rMBP. Here's why:

• Despite being called a laptop, a 12" retina screen becomes hard to read when sitting in your lap. Real estate aside, it's hard on the eyes at that distance. For instance, I typically have to bump google docs up to 125% or zoom in on pages. I didn't think this would be a problem as I used to have one of the old 12" macbooks (that was a decade ago, man I'm getting old) but it totally is.
• It's bouncy in your lap. No seriously, it's so light that typing in your lap is wobbly.
• It really struggles. I got the baseline model and things like google docs and PDFs in Chrome cause it to lag. You'll notice things like the interface stuttering when switching between full screen apps. I haven't done a lot of design or coding on mine (yaaaa grad school) but judging from my experience thus far, you will be frustrated.

Now for the pros:

• It's light, like super light. This makes for a great everyday carry.
• It's small, if you have some big backpack (30+ liters) you could totally downsize to something around 20L and get by.
• It's really sharp looking
• I actually prefer the keyboard, it feels nice and oh yeah, force touch is a gimmick.
• Ok, really, the pros are that it's small and light.

Hope that helps you.

Edit: just noticed you went to NEU, huskies represent! Also, I work right off of Alewife so Cambridge/Somerville represent too!