I switch my system out every few months on CL and have used MBA 11, 13, rMBP 13, 15, and a Surface Pro 1, 2, 3.
Since it sounds like you're set on a Mac, my recommendation is first to decide whether you want a 11/13/15. The weight is a non issue these days. It is the 'bulk' that will be most noticeable. I can fit a 11 in a large purse and is easy/convenient to stow or carry around open with one hand, while a 15 is a snug fit in most backpacks. The 13 I belive is mostly popular due to its price point, but I feel loses the benefit of the 11's portability and the 15's power / screen real estate.
I have a fondness for the MBA 11 and highly recommend it for ultimate portability. It may not seem like it is that much smaller than the MBA / rMBP 13, but the dimensions make it MUCH easier to tow around. I found that I acclimated to the screen dimensions very easily. My only caution is that if you tend to write a lot of research papers, the smaller screen precludes viewing a document and PDFs simultaneously and
I would advise getting a inexpensive 24" display to use at home if it is your only system.
I would recommend getting the rMBP 13 over the MBA 13, but I would caution getting any 13 since while they are the jack of all trades, tey are the master of none. The rMBP 13 does not weight much more and the display makes a noticeable difference on eye strain when doing a lot of reading.
I now use the rMBP 15 because portability is no longer an issue and the larger screen offers more real estate for keeping two documents open side by side. It is great for writing longer papers, 15" I have found to be enough to not really require an external display. The downside is the cost that is the result of the higher performance CPU / potentially GPU / base RAM / base / storage. Most users really don't need will not require the power. Not everyone will need/want this option, but if on a budget 2013 & 2012 models can be had for less than the price of a new rMBP13 base model with a little savvyness on CL.
Specifications: I would recommend 8GB of RAM, but if you are really on a budget you can get away with 4GB if you only do light web use and document writing. More ram will be useful is you start having numerous large PDFs, programs, etc. running simultaneously. 128GB storage is enough to get through your average user needs, and getting a 1/2 TB USB 3.0 drive ~50-80 on sale is the most economical way to go if you are on a budget. If you tend to use more media (photos, videos, music) then I would advise getting the 256GB storage option just for convenience if you can afford it, and again get a low price external for any excessive usage/videos.
Surface Pro. I spent about 6 months carrying around a MBA 11 AND SP2/3, although ultimately I chose to do MB + Paper. SP1/2 are incredibly cheap on CL where I live, and this option can still be done on a budget (~250-450) if you want one just for OneNote. SP3 is larger with considerably better battery longevity, and the stand is useful.
PROS: I enjoyed the concept of the SP. I have yet to see an iPad stylus that truly simulates the accuracy, ease, and effectiveness of a true digitizer input. It was great for physics and math notes, where I didn't spend a lot of time using color. OneNote is incredible, and last time I checked the iPad OneNote misses much of the PC based functionality (such as the pen). EverNote, Notability, are neat applications and I have seen people use them effectively, but the experience is different and the accuracy and speed are missing. SP3 keyboard (not SP1/2) wasn't that bad to use on a lap in a stable place (lounge chair), but was still difficult in a place like a bus, plane, etc. Although I don't tend to write long document in those precarious positions so I always thought that argument was a little misleading.
CONS: It never really replaced the kinetic feel of writing on real paper. That really bothered me and I suspect part of that has to do with handedness. If you are left handed you 'push' your hand/pen across the screen, as opposed to 'pulling' if you are right handed. I found the 'pushing' to be awkward with the glass surface and never get one of those gloves to use while writing. It is also a bit of a hassle to switch colors on the fly while note taking. I am not a Windows person anymore and anything outside of OneNote was awkward and clumsy, so I felt the need to still have my MacBook.
PS: AppleTV streaming is incredible for YouTube, Lectures, etc!