Portofino is DEFINITELY worth the extra cost! We had a fabulous meal, with service to match! I highly recommend it! I would do it again in a heartbeat.
(We scheduled our Portofino dinner for the same time that our normal dining time would have been - 6 p.m.)
p.s. Definitely watch your cruise compass for the showtimes. We didn't realize that on one evening they were only offering one show, and we arrived just as the theater was emptying.