They're both DT's. They're definitely comparable players. We're not talking DT and CB here...
Fairley is pass-rushing DT and they lost out on him and immediately turned to Knighton, who by your admission is a run-stuffing DT, so clearly the Patriots see them as very comparable.
Knighton's a 1-technique and Melton's a 3-technique. They're 'comparable' insofar as one could be used to play the other position in a pinch but they don't fill the same role. Fairley has played both 1- and 3-technique over his career, including with the Lions when Suh played 3-technique and last year where he mostly rotated for Brockers though played both positions.
Based on these visits, Belichick clearly thinks 1-technique is a place that needs to be strengthened, since Hicks and Siliga both filled that role on last year's squad after Easley was hurt and Brown became entrenched as the starter at the 3-technique, since he can play all along the line. The Patriots also signed Frank Kearse, who I know little about but is said to have versatility to play the 3-technique.
The current depth chart looks like this (I'm only putting this in the base 4-3):
UT: Easley, Jones, Kearse
NT: Brown, Branch, Kitchen
Brown's got versatility at both positions and will play NT if Easley's healthy, but who knows if Easley will ever be healthy. Without Easley, the depth chart looks like:
UT: Brown, Jones, Kearse
NT: Branch, Kitchen
Probably an okay rotation at the UT (Jones is at least replacement-level), but the Branch/Kitchen rotation is a significant step down from the Hicks/Branch/Siliga rotation we had last season. There's certainly room for two rotational defensive tackles, but unless you're comfortable with Kitchen playing 20%+ of defensive snaps this season, the 1-technique is definitely the more pressing positional need. Hence why Knighton and Fairley have been the targets.
After Knighton and Melton, some remaining defensive tackles are:
UT: Jason Jones, Jason Hatcher, Randy Starks, Mike DeVito, Antonio Smith, Billy Winn
NT: Tony McDaniel (has played both), Kevin Williams (same), Cam Thomas, Kyle Love, Dwan Edwards
The cupboard is pretty bare.