What is the difference between the two?
6EXA and 6EXP the 72Mhz version.

Not much, don't buy new 72 mhz, there is a huge amount of it available used, dirt cheap. If you going new, check this out [link]http://www.hobbypeople.net/gallery/751700.asp[/link] Thats a whole lot more radio for the money.

Not sure. I have the T6EXAP which is actually just an Aircraft Radio in it's form, but I think the EXP is an Airplane/Heli radio. I don't see the EXA. I think mine was marketed as the EXA.
Good radio though, I have enjoyed mine for what it does. It is nice to be able to pick up some good used Receivers now for cheap.
Curtis

Here is the info on the 6EXA

http://www.futabarc.com/discontinued/futk55.html

And here is the 6EXP

http://www.futaba-rc.com/radios/futk62.html