I would but I'm still on the SERO plan and I'm too cheap to give it up.
This is a tricky one - The N6 is a great phone. I use it, and will likely skip the upgrade this year (I usually upgrade every year). But if I didn't have the current N6, I'd wait another month or so to see the new Nexus devices and get one of those instead. One extra year of guaranteed updates - why lose that (along with hopefully better/newer hardware)?
Regarding the LTE speed on your Note 5, contact Sprint support and tell them. They will likely push out a forced profile update (which, I'm told, is somehow different than the profile/PRL updates you can initiate from the device) to your phone. See if that helps. The Note 5 is a Spark enabled device, so assuming you have Spark coverage in your area, you "should" get fairly decent speeds.
(Disclaimer: I was in a Spark enabled area from when Sprint launched Spark, and ALWAYS got crappy speeds on my spark enabled phone. Lousy LTE speeds too, even though signal strength was "good". But that was for any/all Sprint devices - didn't see better speeds on one device compared to another)
As a former SERO holdout, I feel the pain. Put up with lousy service for too long just to save a buck. Then, joined up with a friend and got a T-Mo family plan. $30 per line (less than SERO) - 10GB of actual usable high speed data, and unlimited everything, with international (low speed) data for overseas travel. Sweet! Their 3g speeds (in areas that I visit) are better than Sprint's LTE. YMMV