1) I would not buy a new HTM or Harmony remote off of ebay, both those companies have an authorized dealer policy, and that means no warrenty

2) I like PC programmable remotes and inyour list that leaves the 3000 and the 688.

3) From what I heard in the Harmony forum more people tend to like the 680 to the 688 on the other hand I have not used neither of them

4) if you want a HTM remote you can find the MX-700 or an open box MX-800
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some brake downon the remotes you chose

Pronto
- pro
... most custumizable remote
... you can have a lot of fun with it (can do more then remote stuff- just look at the pseudo games some developed for the 1000 and the 3000 has more capabilities)
... can be extremely easy to use if designed properly
... looks cool
- con
... you need to learn ProntoEdit (not hard, but you need to learn how to use it
... it is a blank slate, so you need to know what you want
... can take many hours for original work
... it is almost a new hobby and it overwelms some people

Harmony
- pro
... easy to set-up and web based
... Harmony support (fast and good, and if it is a real tough problem they will do everything they can to fix it)
... you can get an electronic programm guide if you want (I think it is 20$ or something like that) so you can click on the name of the show to pick it
- con
... no in built RF (not much of a con, but the mx-600 comes with an RF base and it is anaccessory for the 350 and pronto)
... you need internet access to edit the remote

MX-350, 600, 500
- pro
... good remotes and well liked
... 500/600 can be used with IR-clone and that makes them PC programmable
... 600 and 350 can do RF to a base
- con
... not PC programmable
... the 500 is IR only
... least cusomizable of all the remotes mentioned
... LCD screen is 5 letters/button

MX-700,800
- pro
... mucch more capable than the 500/600/350
... PC programmable
- con
... the 500 is IR only
... LCD screen is 5 letters/button