Hi Leon,
I have both HEQ5 and AZEQ6; AZEQ6 is sturdier and seems to have more horsepower too. Not a massive difference is tracking accuracy though, given you stay within both mounts capacity.
Check page 19 in both manuals for more detailed info:
AZEQ5:
http://ca.skywatcher.com/upfiles/en_...1440636363.pdf
AZEQ6:
http://ca.skywatcher.com/upfiles/en_...1353096919.pdf
Hope it helps a bit.
Regards
Suavi