Since writing this though I realised something important I had forgotten - these are rechargeable cells. And if they are wired in series, it means I need a much more complicated charging circuit (I currently use TP4056 boards to charge cells).
So even though series might be more efficient, I think I'll need to use parallel as a single TP4056 can charge two cells in parallel.
Unfortunately after posting this I discovered the TP4056 board won't charge 18650s in series, only in parallel!
Since writing this though I realised something important I had forgotten - these are rechargeable cells. And if they are wired in series, it means I need a much more complicated charging circuit (I currently use TP4056 boards to charge cells).
So even though series might be more efficient, I think I'll need to use parallel as a single TP4056 can charge two cells in parallel.
Thanks for the answer though!