I think it would sort of depend on the conditions. If your not going to do them very often, then the Polish would be the one to go for, my mom was impressed with the difference this made to hers after they were done with it for about the first time in a year.
If used regulary like every week or fortnight like I tend to do, then the Fast Glass will probably do the trick.
Might be better to go towards the Polish as you could also give the wipers a wipe down properly then too, as Rich says above.
Both the same price anyway I think, about a fiver.
Chris