Reverse osmosis/deionized water and distilled are in a nutshell the same thing. Its just 2 different methods of producing pure, mineral free water. A steam distillation apparatus is used to produce "pure" water, there is probably a filtering step in commercially distilled water somewhere. Reverse osmosis water is created by forcing water through a semi-permeable membrane that traps most foreign bodies, the deionizing step forces the water through a charged resin that removes any left over chemicals that RO
doesn't catch. Most commercially sold RO
water is also subject to physical filtration and activated carbon filtration. For aquarium purposes I would say that both are equally pure, the only catch with distilled is the the piping used during the distillation. Some worry that copper pipes used during the process could produce water with too high of a copper content that could harm inverts.