In for a penny, in for a pound. Stainless steel is well worth the investment (IMHO).
I have never brewed in aluminum. It is much more reactive, chemically, than stainless steel. That gives me pause...
I was fortunate enough to have a wife who gifted me with a Vollrath 10 gallon SS stockpot early on in our marriage. That worked great for a long time, for 6-7 gallon batches. Then I moved up to 10 gallon batches and started using a converted 1/2 barrel sanke keg for my brew kettle.
The 10 gallon stockpot makes an excellent "open" fermenter for my fruit beers and meads.
Go with stainless as soon as you can afford the investment.