Can't comment on the BMW but I have an E-Force BRZ. Not as refined by a longshot (tricky 3-puck clutch/lightweight flywheel chatter and stiffer suspension don't help) but upside to that is it's very visceral.
If I didn't already have a BRZ, I'd seriously consider a Porsche Certified Pre-Owned Cayman. I've gone through the motions of switching to one (ultimately prefer a 987 over a 981) but couldn't justify it as the E-Force BRZ feels about as fast but better viscerally.
Some of my thought process:
http://www.ft86club.com/forums/showp...&postcount=618
http://www.ft86club.com/forums/showp...&postcount=216
As far as E-Force BRZ tS vs 987/987S Cayman... I prefer the looks of a BRZ tS but MR layout is more elusive and factory build makes it more stout, however, it'll be used. Tough call. I'd also consider a base 718 (comparable to 981S).
But if you won't consider Porsche for whatever reason, disregard everything I said except an E-Force BRZ tS will not disappoint, especially if you liked your first 86. Best to test drive both configurations you're considering.