I wrote an extensive review of 500 on amazon where I basically came to the conclusion that unless you require wifi, there seems to be very little reason to get a 500 over a prohd. Overall, I gave it a 2 star rating, mostly based relative to the capability of the prohd. If the prohd never existed, it would probably be higher. You can find the review under "Most helpful critical review".
In a snapshot:
Quality was virtually indistinguishable (same?) between component of prohd and hdmi of 500 (and I was one of the few people who didn't have a problem getting the HDMI to work). Anyone notice a quality improvement? I didn't.
ProHD also allows use of stand alone slingplayer, the venerable slingcatcher, and truly has multiple inputs (including a digital coaxial audio input). These are all considerable advantages over 500. As for the 350, I haven't tried it', but I would think the general agreement would be that the ProHD wins out(ProHD just does more). But not having tried it personally, I would like to hear other opinions.
I would add that in many countries, the ProHD is still the current flagship model (including Japan & UK).
There may be some future capabilities of the 500, that are not yet exploited in the current firmware. However, until/if/when these are developed, my vote is strongly for prohd over 500.
I would support those answers, but I would throw a question back. Why change the Solo? If you are only using a single composite or component output then the quality will be just about the same. The only advantage of the Pro-HD is the ability to input 2 different Audio streams and the built-in tuner (which has been discontinued. In the 350/500 so I assume that isn't a requirement?)
My view having run a 350 and Pro-HD alongside each other for several months is that picture quality at the highest setting of the 350 is both smoother ans sharper than the Pro-HD and seems to take a different approach as bandwidth fluctuates; the Pro-HD drops frames and seems to judder in fast moving scenes or sports whereas the 350 seems to retain the framerate but pixelates/becomes blocky temporarily. I prefer the new approach.
However aside from that the Pro-HD offers many advantages over the 350 and it's hard to justify recommending the new boxes unless 1) the improved video quality is important to you or 2) the wifi feature of the 500 is important to you.
They've removed a number of useful features both in terms of configuration in their quest to simplify the boxes for the masses, and by not supporting the desktop Slingplayer and the Slingcatcher they've essentially removed support for many people's favourite ways of watching their Slingbox! So you are forced to use the web player and connected device player both of which in my opinion are inferior to their original counterparts and judging from the comments on these forums I am not alone in this belief.