The full frame mirrorless many recommend like the Sony a7/a7r is deceiving because though the body is small, the lenses are literally still as big as DSLR full frame lenses - in fact the 55mm f/1.8 for the Sony A7 is twice the size of the Canon 50mm 1.4! So you have this tiny body but still have to lug around the big lenses making the ordeal pointless and an ergonomic mess IMO.
Your general point may be right, but the Sony/Zeiss 55mm 1.8 isn't twice as big as the Canon 50mm 1.4 - it's a bit longer (71mm vs 50mm), but somewhat narrower (64mm vs 73mm) and weighs slightly less (281g vs 290g) (data from dpreview as I'm too lazy to measure mine, but it seems about right). It's also optically superior to the Canon, though of course not quite as fast (if you don't mind manual focus, the Canon, by the way, works very nicely on a Sony A7/A7r, the mirrorless technology avoiding the back/front-focusing issues often complained about with this lens).
The Sony/Zeiss 35mm 2.8 is tiny, almost exactly the same size as its micro 43 equivalent, the Olympus 17mm 1.8 (both weigh 120g, there's a whopping 1mm difference in length and 4mm difference in diameter). It's optically superior too.
As for the original question, it depends on what he wants to use the camera for and what he wants to do with the resulting photos. If weight and bulk matter a lot and lens versatility matters, micro 43 wins out - optically much the same as most APS-C, the lenses - some of which are excellent, esp. the closest equivalent of the 135L, the Olympus 75mm 1.8 - are all much smaller than dslr equivalents, and if you buy an Olympus body, you get excellent image stabilization for any lens you attach, which makes up for at least some of the low light disadvantage these bodies have cf FF. Whether what is perhaps the main difference - relatively longer depth of focus - matters is a matter of taste (the bokeh can be superb, though).
If you don't mind the various compromises involved with the Sony A7/A7r (shortage of native lenses, not designed for herons-catching-fish, etc.), the image quality is unsurpassed (except perhaps by the 6D and 1Dx in very low light), and they're small and light.
If you like playing around with old manual lenses (or new ones, for that matter), on mirrorless bodies focusing is much easier than on any dslr. AF is better in at least some respects too - you can move focus points just about anywhere in the viewfinder (especially beneficial if you want to avoid the problems that come with focus-recompose) and there's no issue with front/back focusing.
Or a high-end point-and-should may suffice.
Anyway, they all have their pros and cons. I used not to mind (much) carrying around FF dslrs and a lens or three, but having spent the last few months using an A7r, A6000 and Olympus OMD-EM5 almost exclusively, my 6D and 5DIII feel terribly bulky and heavy on the few occasions I've used them and I just don't enjoy the process of taking photos as much; for now I'll keep at least one of them, but I'm not sure for how much longer. We're all different, though, so it's hard to make any sort of specific recommendation.