I would go with what the manufacturer recommendation--the only difference in price is the cost of the GFCI breaker--50amp and 60amp wire is the same guage which would be #6 copper which is rated for 60amps or less. You can only load breakers to 80%of the load rating eg. 15amp breaker one can load it to 12amps. eg. 60amp breaker loaded to 48amps etc...thats the way we do it in Canada.
Check the nameplate rating for FLA (full load amps) on the tub and go from there. My Arctic Yukon has as FLA of 30 amps therefore to run all options i need a 40amp GFCI.
Thats the way we do it in Canada.
Hope it helps you out
Hope this helps