If we're talking about the latter portion of the season, I'll say Colvin, but if we're talking about the close Super Bowl in which a few plays here or there could have been the difference between 19-0 and 18-1, I'll buck the trend and say Morris.
My reasoning is that Adalius Thomas had an outstanding Super Bowl playing in Colvin's spot. Even when he wasn't getting his two sacks, he was putting pressure on Eli or bull-rushing the tackle into the pocket. The AD in SB42 was the AD we wanted to see all year. I don't think Colvin could have done better. As for the ILB rotation, AD started off the year well there, but started to slip and often found Bruschi and Seau getting more of the reps anyway. Seau was the best run-stuffing ILB of the three, definitely by the end of the year. In a title game that the defense played well save for some dropped INTs and a circus catch on that final drive, I don't think AD rotating inside with Colvin outside would have kept the Giants from scoring less.
Now onto Morris. Maroney finished the year strong, especially against San Diego, but he was able to either sweep wide or have huge cutback lanes thanks to some outstanding blocking. In SB42, every time he tried to take something wide (3rd and short, anyone?), the Giants were ready. If you remember earlier in the year when Morris played, it was often he who would finish out the blowouts by killing some major clock, grinding out first downs up the gut when the defense knew he was coming. He was more of a quick-hitter which against this fast DL is probably what the Pats needed more. He probably would have softened up the Giants front a bit more than Maroney, maybe either slowing the rush and/or letting the OL get an edge in the trench battle. I'm not saying he would have run for 100 yards on the Giants, but his running style of hitting the hole hard and fast might have tipped the LOS battle just enough to give Brady the second more he needed on a handful of throws that probably would have been the difference in the game.
Regards,
Chris