Note that the 'guaranteed' upgrade on every rental is certainly not guaranteed. There are many threads here about how PC often doesn't get us any better service or any appreciable upgrades.

Personally, I get an upgrade in about 2/3 of my rentals... definitely not 90%. Perhaps you would have better success in Europe than in the US with upgrades.

Since you have a free offer however, I'd say it is worth trying and seeing how your experience is.