Assuming you're doing full volume extract boils and unless you'll be doing BIAB or building a mash tun you don't need a bigger kettle. The one you're using now for a boil kettle will work just fine. If you just want a bigger kettle so you can step up your batch size or want shiny new toys, there's nothing wrong with that either.
As far as the SS vs Al debate goes, it largely comes down to budget and user preference. An Al kettle will hold water just the same as a SS one, and at about 60% the cost. You just need to be mindful not to use abrasives or certain chemicals when you clean out an Al kettle.
If you keep an eye out and aren't on a strict time table it is possible to find 10G SS kettles for $80-120. I know you mentioned 15 but I don't remember any of the sale prices I've seen for those, sorry.