Personally I don´t like the 90x90 silks since I find them too small and the silk twill too stiff. I have bought many but never worn them since they don´t work for me. I love the GMs, they are soft and big and gorgeous. I think a GM is more verstaile than a silk scarf. Hermès uses special cashmere that will adapt itself to the surrounding temperature. A bit like a termostat (if that is an English word). Good luck for your decision.
Welcome to the dangerous world of shawls/scarves
Adding that I bought a GM silk scarf a few days ago which is supposed to be much softer and its size will make it work for me. I´m posting a picture for you to see.