If you have the bandwidth there is no comparison in my view, the newer boxes (350, 500 & M1) give a way better picture than the Solo. Your Solo will send a maximum resolution of 640 x 480 which is standard definition. The new boxes will go up to 1920 x 1080 ie HD.
However you need enough bandwidth. If you are viewing on your local network then your bandwidth will be fine and you'll get HD.
If you are viewing remotely, then it'll depend on the smaller of your upload bandwidth where the Slingbox is and the download bandwidth where you are viewing.
If this figure is at least 3-4mbps you'll get an HD picture. If you only have 1mbps then you probably won't see much or any difference between the Solo and the 500 since the 500 will only send a 640 x 480 SD picture, just like the Solo. It might be slightly better by virtue of using a better compression method encoded by newer hardware, but not significantly. If you have something in between, say 2-3 mbps, then you'll likely get intermediate resolutions like 960 x 540, 1920 x 540 depending on the exact bandwidth, but even these, in my experience, are notably better than what you'll get from the Solo.
I wouldn't get the 500 on the basis of the personal media functionality though. They released this full of promises to enhance it but in the last day or two they've suddenly announced they're removing a whole bunch of functionality.
This is very typical of Sling these days - they release and promote something, then drop it a year later. I wouldn't recommend relying on any features of the hardware other than the basic Slinging functionality.