I would go AC as well and even though "client won't cover T+ fares, so no hope of using my eUps", I might consider paying the additional myself. I have done this in the past when client wouldn't cover First Class or higher econo fares and I personally paid difference between Y and F regularly or Tango and Flex (Tango Plus). Depends what you need out of all this. AC planes in the back are quite comfortable (I actually flew in the back for the first time since the early 2000s in December from Chile). As long as I have an aisle seat and some entertainment, I can endure the throw it at you service, airplane food etc etc. Having an empty seat next to you does help though. But as a SE, if you can swing the upgrade, in a similar situation, I would pay the difference, even if it is a $1000 or so (for status miles and possibility of upgrades). If J is already low availability or zeroed out, I might rethink but likely would still go with Flex.