The short-version is that the tiny little bearings in the aluminum idlers are crap (especially for the $ premium you are paying for them). Flanged bearings are cheaper and (likely) higher quality than the common idlers available from the usual scumbags. I bought 50x F625Z bearings (which make 25x idlers) for something like $8.
An argument for the aluminum idlers is that toothed belts on smaller smooth bearings can cause artifacts, but the popularity and performance of the Voron printers sort of disproves that. A Voron 0 uses F695 bearings and the Voron 2.45 uses F625 I believe. Even consulting official documentation (from Gates for their GT2 belts) they say smooth idlers are find if the pulley diameter is >= 40t is acceptable. I believe the F608 fits that bill if you are really concerned. Perhaps the other argument is that the small flange (only 1mm) on the bearing may allow the belt to escape. That probably isn’t a concern with relatively short belt runs, but for a large machine (LowRider, or a Large sand table) it is something to keep in mind.