I would say the nod to the best overall 2D picture quality (between the Panasonic and JVC) goes to the JVC. Two different technologies... Primarily the JVC doesn't rely on an auto iris and has much better blacks.
The Sony 50 and JVC are more comparable in tat department... The Sony probably has better 3D capabilities and the JVC has an advantage because of auto memory zoom (if you have a 2.40:1 screen you'll want this). JVC also has created the 4k eshift technology that is on the 35.
Bottom line, you get in this price range and the projectors are all really nice. If I were you, I'd make a list of the things that are most important to you (lumens, 2D capability, 3D capability, motion modes, lens memory, etc) and read professional reviews (projectorreviews and projector central are decent starting points).
I ended up with my JVC because I did my own head-to-head, JVC vs Panny, in my theater room. The JVC won out for my needs. Your needs/wants may differ, tho.
I guess you're based in the USA because the RS48 (X55 in the UK) is not just 'slightly' more expensive. In our case in the UK it is virtually twice the price of the X35, especially if you are considering the 2D only option that I bought (£2,500 verses £4,600). The RS48/X55 has other extra features over the RS46/X35 though, so I'll list them in case anyone is considering the options:
RS48 upgrades over the RS46:
Zonal pixel adjustment (in addition to the single pixel adjustment which all models have)
More lens memory slots (10 instead of 5 IIRC)
More screen adjustment settings (99 instead of 3, though not needed if doing a full calibration anyway IMHO)
I just couldn't justify doubling the price over the X35 when I already have a Lumagen for (better than JVC) CMS, a Darbee and an Isco lens which goes some way to increasing pixel density at least for 2.35:1. However, I gather that the price difference in the USA is much smaller (and they are cheaper to start with) so I could understand you guys springing for the upgraded model, especially if you are just comparing the 'bare' projector rather than with the 'accessories' I have.