I'm not so sure. Scotty seemed lonely and looking for love at times (Carolyn Palamas, Mira Romaine, Uhura?). O'Brien and Keiko may have had some marital problems, but they stayed together, seemed to have real love and had a child. O'Brien may have pondered on it during his troubled times, but in the long run, I think he is happy to be married to Keiko. Plus he built a great friendship with Bashir and gets to play darts and drink beer in the bar. Maybe this is why he seemed tried on the job due to off-the-job family plus play time. And I don't think he had any real love for DS9; all it did was break down due to shoddy Cardassian construction. But staying on topic, I still voted for Scotty as the better engineer even though I still liked O'Brien.