Ditto on the Progressive Industries. I had the internal one installed. Did not like the idea of someone being able to steal it off the post. Some like the ones you plug into the post because they can take it and walk around testing power posts.
I went with the Progressive Industries because it has a lifetime warranty and they are said to have great customer service. They are also modular design so if hit takes a it and fries, you can replace the card and not the whole device.
The 50 amp will work on 50 and 30 amp if I use my "dog bone" adapter from 50 amp to 30 amp but you are correct it does not work the other way around.
A rule of thumb is if there is 1 A/C unit, 30 amp will work. If you have 2 or more A/C units you need 50 amp service.
30 amp service is 1 line at 30 amps. Think heavy duty extension cord.
50 amp service is actually 2 lines at 50 amps each. Think 2 super heavy duty extension cords twisted together in into one fat cable.
http://www.womenrv.com/forum/viewtopic.php?f=7&t=11244&p=148104&hilit=surge+protectors#p148104 This might also be helpful.