I don't have an issue with people who believe Demar has been better this year, because I suppose you could argue Demar has followed in the path of guys like Nash, Kobe, arguably Lebron himself, Bird, etc. as guys who peaked offensively in their 30s.
That being said, if the question is asking who on a per-possession basis, has been more impactful, I would go Lebron.
Their basic scoring numbers:
Lebron
Inflation Adjusted-28.9 pts per 75 (rTS% of 6%)
Demar
Inflation Adjusted-28.9 pts per 75 (rTS% of 4.1%)
So even with Demar's recent historic scoring streak, they are scoring at the same rate, however Lebron is doing it on better efficiency. Therefore you could argue that Derozan's current scoring numbers might overstate his true scoring prowess a bit. Furthermore, Lebron keeping up this rate doing while playing more minutes per game (mostly because his team needs him out there), though Derozan has missed less games; so depending on how you treat injuries there is that to consider.
Then Looking at the basic metrics
LBJ
Backpicks BPM-5.8
LEBRON-3.65
RAPTOR-5.8
EPM-6.9
Derozan
Backpicks BPM-4.2
LEBRON-1.95
RAPTOR-3.1
EPM-4.3
Lebron comes out looking more impactful on a per-possession basis than Demar in the major metrics I have listed. One thing that probably should be noted is that for all the backlash Lebron may/may not deserve to get on defense, he comes out looking considerably better than Derozan generally in the metrics.
Now with Lebron's missed games, it is notable to point out that perhaps Derozan has had to take on a heavier load and he should be credited for that. However, if you look at the cumulative impact numbers:
LBJ
EPM Estimated Wins-10.1
LEBRON Wins Added-5.8
RAPTOR WAR-6.6
Derozan
EPM Estimated Wins-9.7
LEBRON Wins Added-5.5
RAPTOR WAR-5.8
Lebron generally stills come out with a lead over Derozan. We know Lebron is a better playmaker than Derozan, and it does look like he has scored better this year just looking at the bare numbers. Statistically, he looks like a better defender than Derozan, and I would say with his occasional rim-protection and orchestrating of teammates of where they should be on defense, it makes sense he would be ahead. Derozan's case would have to lie in the belief he has some gravity that might be hard to detect, although I would like to see evidence that NBA teams give Derozan attention in an especially unique way that Bron is not given. There might be other qualitative arguments that Derozan is doing things for his team on the offensive end, that Lebron is not doing, but I am having trouble at the moment seeing what it could be that would give him the edge here. Perhaps Derozan's ability to operate out of the midrange gives him some qualitative edges, I am not considering?
Part of the intrigue of midrange shooting is that it is ideally resilient against tougher teams as it is seemingly always available. However, as this chart shows, there isn't really enough evidence that Lebron is really great against top defenses himself this year (chart is as of Feb 16. 2022).