One key difference is that the FT-847 can do full duplex on VHF/UHF, which is very helpful for operating on satellites! Most rigs including the FT-991/A cannot do full duplex.
73, Ed VE3WGO
ps. I had an FT-847 long ago, then a TS-2000X, and now an IC-9100. I like the 9100 the most of all.
$1200 for FT-991A. $2100 for IC-9100.......yikes!