Geographically, 3rd Ave. doesn't really run straight up from Union Square - but Park Avenue does, and then you can swing over to 3rd when you hit 46th Street, if you decide on Sparks, which isn't a bad choice, especially if you want to spend some dough on wine. Don't forget, Paul Castellano was gunned down right in front of the place, along with his driver, so you'd get to see some famous sights too!
But - I think the best steaks being served in NYC aren't necessarily being served in steakhouses. Which is probably good for your seafood eating friend (s). This Korean restaurant called Cote is doing lovely meat stuff, but it's a pain in the ass to get a reservation. But my favorite would have to be Frenchette, where the rest of the menu just rocks, the wine list is great, and the beef is awesome. Easily enough for 2 or 3.
Back to steakhouses - Wolfgang's, Keen's (which is a classic for sure), S & W's, Old Homestead, will all get you where you want to go.
