I just did a highly scientific survey of GB 36 Classics for sale on YW. US boats only.
Equal number of singles and twins for sale. Did not track condition or features.
GB 36 single: mid 80's, mostly lehman 120's, avg ask price - $85,000
GB 36 twins: some late 80's, lehman 120/135's, avg ask price - $120,000
GB32 single: variety; mostly lehman 135's, avg ask price - $74,000
Sparse listing of cruise speeds but 7/8 knots common with max of 9/10.
One report of 12 knot max with a Cat 325Hp.
Conclusion: Equal number of each sold for GB36 in the 80's.
There's probably a 10-20 % premium for twins
I can't find any empirical data indicating there is any significant performance advantage so I am guessing the decicions are based upon emotions.