nor'easter, I can't speak for anyone else but I would choose EQ5 over EQ6 because EQ6 has a limitation of how many times you can install it (4 I think, but don't quote me on that just yet. I just remember it's a low number.) EQ5 doesn't have that restriction.
If you've ever had computer problems and had to re-format your hard drive, the installation restriction is something to consider. Personally I reformat every 6 months or so, or if I'm having an issue. I feel that if I purchase the rights to use the program, then the number of times I install it shouldn't be an issue - but that's JMHO. :-)