The 753, 763 and 773 all are about the same power rating. I think the 773 is vertical lift? more pivot points to keep an eye on. The 763 has a lightly longer wheelbase too.
They are all good machines, the only suggestion i can give you is to steer away from any with hand controls. They cause more trouble than they are worth. If its a new machine, sure, you have warranty. If its used, you stand the chance of having problems and LOTS of money.
They are good and strong, more than enough power for their size. Easy to maintain on your own and repair problems if you have any. Parts are easily available for all of those models.