I went stainless for a couple reasons. Most importantly to me is the sapphire glass. It's more durable than ION and I tend to smash watch faces. It's heavier which is nice to remind me something is on my wrist. Stainless also goes with all the available bands where it seems aluminum is more limiting.
As to the cellular question, it doesn't become one with the stainless. But if you go aluminum, having cellular capability is nice to have for emergencies if you don't have a phone nearby. It shouldn't have to be on a plan for that either. If you do provide service to the watch, It's nice being able to get out of range of your phone and still take calls, send/receive messages, and if you want to stream music you can do that too.