Quoted endurance for Pred-C/Avenger is 20 hrs with 'standard' fuel, compared to 14-28 for Pred-B/Reaper depending on loadout.
Avenger has a ~$15M per copy pricetag, compared to, supposedly, ~$30M for Reaper.
Lack of a prop, as well as designed-in stealth features and a higher operating altitude, makes it harder to detect.
One might suppose that improved production techniques and maybe greater component commonality (with other a/c types) accounts for the cost saving?
Still, as long as the raf are locked into their usual timid attitude of go-with-what-you-know, that's just dandy.