Piggybacking here:
Would it be wiser to upgrade to a single 8gb stick in case I need 16gb later down the line or to move to 2x4gb and just hope that 8gb total will be enough?
I bought a 2x2gb 2012 model used and I upgraded to an SSD, but it's time to upgrade the ram. I've got 8gb in my MBA and sometimes that is pushed to its limits
I'd much rather have it and not need it, then not have it and want it/need it.
Every PC i've used over the last few years i have gotten with a middle ground amount of ram, if 4 is the min for most games/applications id go 8, now 8 is starting to be the min for some things so i went 16 for my last Windows PC. Now i have my 09 mac pro, it arrived 1-2 weeks back, before i even turned it on i opened it, put in my SSD, harddrive, and 32GB of ram, then worked on getting my GTX 680 2GB to work in it
So yes, i would much rather have x1 8GB stick, then 2x4's. Think of it as future proofing for you, or if you sell it gives the next guy the option to put in another 8 and call it a day rather then have to sell/discard the 2x4's and THEN get 2x8's