depends
I would recommend a 3" shell cause the payload is way bigger, which means denser patterns, usually. There are some 3" guns that like 2.75 shells better, I hear, so again, shoot some patterns and see.
Nitro could mean several things I guess. There is (was?) a Nitro shotshell company that made "designer" turkey loads, I was given a few, but know nothing else.
Nitro Mag was a Remington Trademark and shot shell line, which I think is still produced. They were lead/buffered and might be different now.
I have not made the jump to tung/space metal shells. The cost seems prohibitive, and I am having no trouble killing birds w/ lead shot inside 45 yds, hopefully half that. I do not want to have to buy a new tube either.
I ran into a closeout at Wally World this summer and likely now have a lifetime supply of Win, 1-3/4 oz ,3", #5's, lead.