i'm about to buy some B+W UV MRC filters for 24-105 f4 and 70-200 f2,8.
I know that it's highly recommended to get "slims" for lenses up to 20mm due to much vignetting visible on FF bodies and even APSC, but the filter price is much higher (~80$) and i only have 7D and 40D right now.
My question is: will the regular filters be good enough or should i get the "Slim" versions for these zooms?
Also: if attached to FF body should they be upgraded to "slims" or will they suffice?
If you're using an APS-C body, no worries about vignetting with EF lenses. Still, I'd get the 'right' filters just in case you do go FF someday - the lenses are long-term, and it's cheaper to buy a filter once than twice for the same lens.
But, for the lenses you mention, no need for Slim mounts. Even the wider and faster 16-35L II can take a standard mount (5mm) filter with no increase in vignetting. So, the F-Pro mounts will be fine.
If you ever do think you need a Slim filter, get the XS-Pro mount from B+W instead. It's 0.4mm thicker (3.4mm vs. 3mm for Slim) and it has front threads so you can use a regular lens cap (Slim filters lack front threads, and you must use the slip-on cap that comes with the filter).
Hoya UV Super HMC seems better to me, blocks less light, has more coatings (12>3) costs the same, only the ring is thicker (5mm>3mm). Or am i missing something?
The high-end Hoya filters are optically equivalent to B+W. Although I have not used Hoya filters, some people have stated that the Hoya filters are harder to clean than the B+W MRC coated filters. Reportedly, the new Hoya HD line is much better in that regard, similar to the B+W MRC or Nano coat. Also, the B+W mount rings are brass, vs. aluminum; the brass has less of a tendency to get stuck (but get a pair of plastic filter wrenches, just in case). I think you're fine with either B+W or the high-end Hoya.
Do we, in that case, only need 'glass filters' for protecting our lens since dslr sensors are not sensitive to UV light? If so, why is the most common filter a UV? What would be the best filter for simply protecting the lens?
Yes, for a dSLR a UV filter is no different than a clear protection filter. As to why UV filters are cheaper, mainly it's market-driven - film shooters were used to buying UV filters, UV filters became popular, etc., so they just sell better today than clear filters. You see that here - we're discussing UV filters, not
clear filters, right? In some lines, clear filters are cheaper than UV (I think that's true for some Hoya lines), for B+W they're the same or slightly more expensive. Also, UV filters are more widely available. But again, for a dSLR it doesn't matter which you get.