Real world experience here from a structural engineer in the UK.
Mac Mini Late 2012 Quad Core 2.3 ghz, 16GB Ram 1TB stock hard drive, running Windows 7 in parallels (4 cores shared and 8GB RAM shared). Rendering on Revit 2015 the medium structural model: 1 hour 20 minutes.
Macbook Air 2013 1.3 ghz, 128GB SSD, 4GB RAM, Windows 7, in bootcamp. Rendering on Revit 2015 the medium structural model: 2 hours 15 minutes.
The MBA handles Revit models up to 150mb without blinking, day to day drawing, detailing, etc. For rendering, well it isn't designed for that and to be perfectly honest Revit is terrible for rendering - if you render alot, then go quad core. Vray, Kerkythea etc. use less resources, run great and render better than revit and cope well with dual cores. In Autocad the MBA handles everything I throw at it in - lots of layers, lineweights, hatching and Ordnance Survey maps with lots of co-ordinates.
I split my workload between the MBA 80% and the Mini 20%. At work I hook the MBA up to a 24" Samsung 1080p monitor, when I get home I just use the Mini as it's hooked up to a 27" 2560x1440 screen i got from eBay.
If I didn't have monitors I liked then I would maybe be tempted by the 15" as the screen can be run at 1920x1200 - so desktop class.
As for the retina 13", I see no massive benefit of: cost v power v battery life. The 13 MBA has turbo boost dual cores (albeit at lower clocks) and great battery life, the 15" has quad cores and desktop class resolution. The 13 rMBP has slightly higher dual core clocks and I suppose a smaller footprint but battery life isn't as good as the MBA and I don't see what it can do the MBA can't. If my MBA had 8GB Ram I could run Revit models up to 250-350mb.
If your a student you can use the Autodesk cloud for rendering work anyway so negating the need for a huge chunk of power - use your student ID/Email to sign up with Autodesk.