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Besides the obvious mount and size (and possible weight) difference between the M and R mount Leica lenses. Are there any significant differences between either M or R lenses that might warrant or sway a purchase towards one over the other?
I'm getting that excited tingly sensation just thinking about this...and no, it's not from the medication kicking in.
I don't know whether to clap you on the shoulder and buy you a beer to welcome you to the club, or to try to scare you away and see if I can't keep it from becoming an addiction.
First off, money. Yes, the R version is considerably cheaper than the nearly identical M version. But a perfectly good Super Takumar 50/1.4 is even cheaper still and arguably provides a more harmonious, more classical rendering with nearly as good handling. So, before you spend hundreds of dollars on a moderately fast normal prime, take many close looks at the sample images here and elsewhere to be sure that this lens is worth it to you. If you decide yes, then shop around. I recommend KEH.com--I'm only affiliated as a customer--as they have a large stock and frequently adjust their prices, both up and down. Be aware that luxury enthusiasts and collectors also buy Leica lenses, meaning that their going rates are not a good measure of worth as a photographic tool in a competitive market. Of course, only you can decide what a good value is.
Learn about the differences between 1 cam, 2 cam, 3 cam, and ROM lenses. They won't affect how they work on an A7, so the only benefit of buying a ROM lens over a 3 cam lens would be newer manufacture and possibly improved coatings, but the ROM prices can be double what an otherwise identical 3 cam goes for.
Secondly, variants of the lenses. I don't know about the earlier Summicron R except that Mr. Puts indicates that it was superior to its M equivalent at the time, though superior in what way is not clear. The later 'cron design is used, perhaps with slight changes, in the Summicron M and Summarit M 50's. In that way, they're all at the top of their class--but that class is no longer the best of the normal lenses, rather of the class of normal lenses designed for film. Digital sensors are more demanding, which is why the 'cron M 50 APO and the Otus are on the market. Both of these are clearly superior to their film-era predecessors.
The second Summicron R has a body that works as a hood, even when the hood is not extended, whereas the M is as small as they could make it.
The earlier R lenses use series filters, which are less common than the later "E" styles.
All that said, the Macro Elmarit R 60, which costs about the same, appears to my eye to be superior to the Summicron 50 when shot at f/5.6 and slower. If that's your style, or if your idea is to maximize output from your new Sony, then the macro might be worth a close look as well.
Last, that Leica look. If you get a second generation 'cron R, you'll find that it exhibits the "glow" with subjects that are just in front of the focal plane when shot wide open. Otherwise...well, take lots of pictures, practice at what you struggle with, enjoy the attempt as much as the success, and, from time to time, share a few images with others.