is there a big different between JVC RS50 and JVC RS20

I owned the RS20 and RS60. The biggest difference was the 3D functionality in the '60. The black level on the '60 was slightly better than the '20, but any improvement in that area between the 20 & 50 is likely to be miniscule. If all you care about is 2D then the picture quality of both should be very close. If you care about 3D then you only have one option.

If I were in your case I will try to get one of the newer models...Either the 45,46,4810...besides the improvements in contrast there is the introduction of e-shift, and in these year's models lamps dim slower than previous generations..

I own the RS20, and I have no need to upgrade for a while. And there is no bulb 'issue' with the RS10/20/25/35 that I know of. It was the RS 40/50/60 that had the issue of premature dimming....

Note a number of the earlier JVC projectors including the RS20 suffer from optical block degredation after 1000+ hours, search for 'pink band' on this forum and you will see a lot of complaints, and its expensive to fix (typically failing after the warranty expires). Personally I would not buy an older JVC model because of it. My RS20 suffered this fate which forced me to upgrade as otherwise I was quite happy with it. I now have a RS56 and the picture is better but only incrementally (a bit brighter, blacks a bit better etc.). 3D is the main difference.

I have an RS2 but the color is cartoonish. The 45 may have the lamp issue, but I'm sure the pq is far better. I would go with the 45 at the same price no queation.

Tom