Stainless or aluminum chimney liner
I would consider stainless preferable in every way. However, there evidently are UL listed aluminum liners for venting gas appliances. I'd still put stainless in my house (if I had a gas line on my street.)

Stainless has better corrosion resistance, higher temperature tolerance, higher strength, and some decades of experience (in general, not specifically with flexible chimney liners) of having far more fatigue and corrosion failures of aluminum items than stainless steel items. 

The only reasons I would normally choose aluminum over stainless don't really apply here - better thermal conductivity and lighter weight.