I have a soft case for my GEM Equinox 88 that I purchased 20 years ago. The Equinox 88 weighs 72 pounds. I carried it to gigs for 12 years in that case. Worked nicely, lots of padding. Also, even if you find a case that is not highly padded you can make it highly padded by adding foam padding for inserts. Glue them in, stitch them in, two way tape them in. Adds more protection but not more weight. So I wouldn't necessarily worry about the case protection as you can make it protected with padded inserts, look for a case that can support a heavy keyboard, has rugged handles and quality zippers. If the zippers break (a $2 item) your entire case becomes useless.