the best mtb cages are the cateye plastic ones (BC100). mine pair cost me 10 bucks and weigh 65 grams. never ever ever drop a bottle. if you want lighter - the ravx beto x at 16 grams each are rated for mtb use, haven't tried them on the mtb but no problems on the road bike. people seem to have good luck with the btp mtb cage at 13 grams aswell but they're silly money.