Dalaman Airport (DLM) sits roughly between the two places, which is why both transfers are so short. Göcek lies east of the airport, on the way towards Fethiye. Dalyan lies west, on the Ortaca side, beside the Dalyan river. The distances are almost the same, about 25 km to Göcek and about 29 km to Dalyan, but the Dalyan ride usually takes a little longer: around 35 minutes against around 25 for Göcek.
That makes either option an easy end to a travel day. You can be through passport control, collect your bags and be unpacking well before an hour has passed, which is a real advantage if you land late in the evening or travel with tired children.