I'll start with Mussina. Pitching 18 years for the Orioles and Yankees, he was a product of his talent and health. He won 270 games, losing only 153. Better winning percentage and more wins than Morris. 3.68 career ERA and 2813 career strikeouts, both more than Morris. Out of his 57 career complete games, 23 of them were shutouts. He won 20 games his last season in the big leagues.
Morris was obviously from the generation right before Mussina. The late 1970s and 1980s were the end of the era where a pitcher was expected to finish the games he started. So, Morris finished his career with way more complete games-175 than Mussina's 57. He finished with a career record of 254 and 186. His 3824 innings pitched were more than Mussina's 3562 2/3. He finished his career with 2478 strikeouts and had an ERA of 3.90.
Morris of course, was known to be a great postseason, being the main factor in the Tigers winning in 1984 and the Twins winning in 1991. He also won a third as a member of the Blue Jays in 1992 and received a WS ring despite not pitching in the 1993 series for Toronto. Mussina appeared in two World Series (with the Yankees in 2001 and 2003, with the Yankees losing both times. Its hard to argue World Series rings for a pitcher who pitched for the Yankees. They won three in a row and four out of five before he arrived in 2001 and won the year after he left in 2009. (Braden Looper won a World Series with the Marlins in 2003, joined the Mets for 2004 and 2005, then won the World Series with the Cardinals in 2006.)
Both pitchers pitched on winning teams for the most part, with Mussina's teams finishing with a slightly higher winning percentage (.551: 1567-1277) while he was there. Morris's teams, boosted by his last four years with Minnesota, Toronto and Cleveland were .538: 1515-1301. Both pitchers, for the most part, pitched on good teams, so neither could be confused with a Steve Carlton who pitched on bad teams for a stretch of his career.
In the end, I'm leaning towards Morris, but its still a toss-up. Mussina had a better winning percentage and won more games, which I think means a lot. I find it amazing that the Yankees never won a World Series with Mussina there, especially with all the talent the Yankees had. Morris made his teams better; i.e. Minnesota in 1991 and Toronto in 1992. He was also the ace of those staffs while Mussina was below pitchers such as Roger Clemens, Andy Pettitte and sometimes David Wells. Therefore he drew some pitching matchups in his favor while Morris always drew the other team's number one starter. Both pitchers had outstanding careers and will be on the Hall of Fame ballot the entire fifteen years they are eligible. But, neither are hall of famers.