While I haven't had an oppotunity to drive a Boxster Spyder, I can't really make a personal comparison between it and the 981. I can only agree with Alexander Palevsky (who owns a Boxster Spyder) when he drove his Spyder and a 981* back to back and wrote his impression in the January 2014 edition of Panorama where he wrote:
"As for the $1,235 X73 Sport Suspention? To order a Boxster or Cayman without it is to miss out on one of the best Porsche driving experiences of the 21st century."
*the 981 in the comparison was a base 981 with manual, X73 suspension, PTV.
You can't order a Boxster Spyder any more. But, for at least a short while longer, you can still order a 981 S with X73 Sport Suspension, PTV, Sport Plus seats and normally asperated flat six to get, what I still believe, will be the best Boxter you'll be able to buy. The exception being the possible Boxster version of the track focused Cayman GT4.
You read what Walter said about the flat fours.