Hi Pat! Good questions. I’ve been considering a 100-400 as well, so I’m interested in what folks say too.
Can I start by asking what you shoot? I imagine the type of shooting might impact what the best course of action would be.
As it stands, you have 35 mm of overlap between you mr midrange and tele lenses. To me, the long tele makes more sense even if it’s slower by a stop. If you’re doing landscapes, I suspect you’ll be on a tripod much of the time, and the f/5.6 won’t be a big deal.