The 817 is like a big HT, runs an internal battery (which is only about 25% bigger than the VX-7R one) and 5W TX. The 857 is the big dog mobile HF/VHF/UHF. It has 100W HF, 50W VHF and 20W UHF. The 817 is for backpacking your station someplace, designed to be small and would probably be a royal PITA to use mobile. Although with your FT-8800 practice operating by braille, well, you know.
"Stand your ground. Don't fire unless fired upon, but if they mean to have a war, let it begin here." -- Capt. John Parker