Upgraded my bike recently and found myself debating 5800 vs 6800. Went with the 105 and its wonderful. Ive ridden a friends 6800 a bit and honestly it really didnt feel much different at all. spent much less on the 105 setup and am beyond happy with it.

I also noticed you havent ridden any new shimano in a decade, i was in a similar situation. I came off of 105 5500 from 2002 and at the time there was a bit of a difference between that and 6500 Ultegra, now however, not so much (at least that ive noticed)