It's very true, Inter is in the UCL, and will play Vida in another week, however I agree with you that it will take away energy from Serie A, while Napoli will be more rested and with more energy to focus only on Serie A, therefore the team for me will get much stronger and in better shape, Inzhagi's Inter is a team that has everything to win both tournaments, however consistency is something that the leading teams in these tournaments of all leagues are suffering from.
Inter does have a tighter match schedule besides having to compete in the UCL they will also face Lazio in the quarter-finals of the Italian Cup today, of course this will drain the fitness of the players, well here Inzaghi's ability is shown how he manages the team to be stable in undergoing this tight match.
On the other hand, Napoli is only focused on Serie A, this will be very beneficial for Conte to prepare the team for the big match next week and regain the top position.
In my opinion, apart from the fitness advantage, Napoli also has the advantage of playing at home and they must make the most of it.