They are both fantastic, and can accomplish the same goals very quickly. I used a great book to learn a while back, and it would be right up your alley: Rails for .Net Developers
Also, I wrote a post about going from MVC to RoR a while back as well.
As for comparisons, they both have strengths as has been covered, but here are a few that stick out to me:
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I really do like RoR’s deployment and simple ways to push to live sites – no scripts to run at all, just a bit of command line should get you up and running once the code is actually written. If you choose RoR, UNDERSTAND THE COMMAND LINE COMMANDS – it will make your life so much easier!
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As you mentioned, the plugins are great, and can really be used to speed things up.
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As a .Net dev, MVC will be a breeze, I would recommend using Razor, as it is very simple and less cluttered than the old 2.0 non-razor rendering.
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.Net’s EF4 can be compared almost directly to RoR’s ActiveRecord, great ORM systems tfor dealing with data.
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I have found that .Net’s documentation is a bit more consistent and thorough.
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One problem I had with RoR was the rapid and drastic changes; not so long ago, they upgraded to 3.0 on my host, and I did not lock my version – all sorts of stuff broke until I figured out the new stuff (I should have frozen my version).
It is not a clean cut choice – both are great. As you are already a .Net dev, I would guess MVC3 would be the way to go, but both are easy to learn.