You have a mistake in your calculations.
Great Crossbows have a crit of "18-20 / x2", not "17-20 / x2".
With IC:ranged Great XBows have a crit-chance of 30% and Repeating XBows have a crit-chance of 20%, bit repeaters have a fire rate of roughly between two or three times that of a great XBow.
on-crit effects will trigger more often on a repeater Xbow in the same time than on a great Xbow. not to speak of on-hit effects.
The advantage of the Great Xbow lies only in the easier aquisition of weapons. How important this advantage will be for your personal play experience dpends mostly on how muchPlat you have.
If you are rich, go repeater. If you are poor go Great Xbow. If you are moderatly endowed there is no clear "better way" for you to choose.