Yep, it does pretty much come down to preference. And like Jeff implies, the biggest difference is in the cleaning. With stainless, you can scrub all you want with something like Barkeepers Friend and keep your kettle looking all shiny and new. With aluminum, you need to "season" it first to build up an oxide layer and then don't get too carried away with scrubbing so as not to remove that oxide layer. The oxide layer may not look as pretty as the stainless, but they both will function well for you.