We did do the Tamur and it was an awesome river. Very scenic, really great whitewater (kind of reminded me of the SF Salmon in Idaho), and just an overall cool experience. We paid for a truck to bring us to the putin because it was supposed to be faster than doing the trek in, and therefore would give us time to explore some of the tributaries of the Tamur. I 100% advocate doing the trek in, the drive was a 30 hour suffer fest. No Humla Karnali for us, but we did get to do the Thule Bheri which was an amazing experience. We also paddled the Balephi Khola, Bhote Kosi, Sun Kosi, Mewa Khola (Tamur trib), Maiwa Khola (Tamur trib), Upper Seti, Upper Upper Kali Gandaki, Upper Modi Khola, Madi Khola, Marsyandi, and the Buri Gandaki.
I don't really have any advice for you on which river I would choose of those two (both if possible!), but I am sure whatever ya'll do you will have an awesome time. I really would like to go back sometime in my life and paddled all the other classics that we happened to miss out on.