That tends to be a matter of team preference. Swampert is mainly used as a bulky Swift Swim Sweeper for Rain teams, since he can take hits decently well and gets pretty good coverage with his STABs.what's better between mega swampert and mega sceptile in OU?
Sceptile is more of a fast Pokemon played for either hit-and-run or cleaning thanks to his very high speed and decent offenses. Sceptile makes a very good anti-offense mon, but his typing leaves him with a lot of weaknesses that mean he has to watch out for bulkier attackers and priority.
The two don't tend to fit on the same types of teams, in that you won't find yourself building a team and then needing to fill a slot that either of them could do.