19" isn't an option on the GTS.Look up a recent Porsche, say a 2016 Cayman, and go through the motions of obtaining its free valuation from a site like kbb or Edmunds. One of the pages will ask about colors, and usually the most popular colors are on top. So if one of your choices has broad appeal and the other doesn't, then that could affect your resale value.
I'd choose the 19" Boxster S wheels. I think the thinner spokes look more elegant than the Cayman wheels, but that's just my opinion. HOWEVER, go for 19", not 20." That will give you many more tires choices.