What's popular around here are those worthington composite tanks.
A thin steel tank *wrapped* in carbon fibre for extra strength.
they are much smaller than ALU (size of a steel), and very light. and are rated to 300 bar.
There are different sizes 7, 10 and 12l, but IIRC one of them is just too light (floats with +100bar) I think it was the 12l?, the others are better.
drawbacks:
expensive (~300 euro's?)
somewhat fragile. rumor goes if you damage the wrapping they will fail your tank on hydro
I don't use them myself but are very common around here, I just carry twin 12's or 18's on my back for bail-out
