First - if you don’t use your 3qt saute pan, you aren’t likely to use a 6qt saute. So I’d say, no you probably don’t need it. Unless . . . you don’t use your 3qt because it is too small for what you typically cook (or for how many people you cook for). But that doesn’t sound like the case from your post.
I have a few large saute pans that I use when I’m cooking for crowds. I could probably do these same dishes in a fry pan, so if you have fry pans you use then maybe you’re covered from that angle as well.
But I use my saute pans (most commonly) for anything that I “sear-deglaze-simmer” and for things that I braise uncovered, where I still want surface browning. I also use my saute pan when I’m doing risottos because I find that I’m less likely to accidentally swish things out of the pan when stirring.
As far as AC types . . . I’m no expert but I have a mix-match array of AC pots and pans that I have accumulated over the years. For my cooking, I find very little difference among the various permutations - so for me the choice is purely aesthetic at this point. I don’t think a copper core (for example) cooks any differently than the standard 3-ply . . . .
