In the market for a Cron

So recently after selling a good deal of cameras i have acquired enough money to purchase an older summicron for my Leica m6. Im super excited and my options are currently between the rigid and v3 50mm. I heard the rigid is far more sharp and i would have loved a DR but from what i hear it cant be put on any newer leicas which eliminates the posibility to expand on the camera if i ever sell my soul to purchase a m240. So with that in mind what are the differences? Which is sharper? What are the pros and cons of one over the other?

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I prefer the rigid to the v.3 and the v.4 to both. They are all excellent lenses, and the differences I'm going to cite are me being nit-picky. The v.3 is slightly higher in contrast but seems slightly lower in resolution away from the center of the image. if you shoot anything but the finest grained film you probably won't notice anything but the slight contrast difference. The v. 3 seems slightly better wide open, but I almost never shoot at f/2.

I don't see why DR can't be on newer M bodies. It can (100% sure) without goggles and I'm just lazy to google it for goggles on digital M bodies. Why? Because new (digital) bodies have exactly the same RF, VF as old ones.
I googled a lot about 50 Crons while getting one. I've had v1, which is underdog among some Croniacs, except radioactive LTM version of v1. And even my not the best copy in terms of optical condition was SHARP. All 50 Crons are sharp enough, DR (Rigid) are slightly more sharp.

I changed my v1 to Elmar-M 50 to have lens from nineties, not form fifties. Many old Leitz lenses including DR(Rigid) have issues with optics. Separation, fungus and scratches due to very soft glass. You might pay extra for flawless lens. I paid for Elmar-M much less, the optics are modern, the rendering is special and classic.

I use the DR on my MP with no problems. Perhaps my favorite 50 of all time.

I have v3. After somebody wrote that resolution is slightly less that DR - this lens become best buy summicron. It is a superb lens with great resolution and contrast - you can not go wrong with any summicron that is in good condition (more important than which version should be condition of the lenses that you can buy on the used market for the money you have).