Moderna seems to be slightly more effective but with slightly worse side effects. Dose is the most likely explanation (30µg for Pfizer/Biontech vs 100µg for Moderna).
An additional reason is that Pfizer/Biontech was available first, so old or otherwise vulnerable people are more likely to have been vaccinated with that.
That is, Moderna is probably more effective, but people vaccinated with it have an even lower death rate because they're less likely to have been in a vulnerable group.
This is also a major factor that can be missed when going over the vaxxed vs unvaxxed death rates. If (eg) 75% of the population is vaxxed, 5% chose not to take it and 20% are too young (and thus very unlikely to die of Covid), then the difference in death rate will be much smaller than you'd expect based on vaccine effectiveness.
