Not sure what the OP's uses are, but I have the 6+ still and it runs fine. I guess from the OP's standpoint he should lean for the 6S+, but for me, the 6+ has been great since day 1. Laggy? Well I don't put my phone through heavy usage enough to see any lag.
I didn't upgrade to the 7+ because the upgrades were too minimal. I kind of wish I had just because it would have been nice to have the updated camera, force touch id, etc, but apple should have made some other design changes to warrant the upgrade.
I am on the fence with the Samsung, but feel that the coolness will wear off and I will be back to the iPhone. Plus most of my family and friends use iMessage and thats convenient. Plus, having iMessage to answer texts on my mac is nice.
OP: go with the 6S+ just to future proof yourself a little longer.