Hello @OP, imo, I prefer the very sack compared to 50k prayer xp simply based on the rewards from the Sack of very wild rewards so I wish to ensure this is safeguarded at all costs and maybe even someday I will get a core after 100+ kbd events (and also all the other events which produce sacks).
Prayer xp can be attained in so many different methods at a rapid clip these days, especially with a powder of burials or the chapel at the fort or (as I do) bonecrushing almost everything in my sight.
That's just how I feel and will always feel even after I somehow get this very elusive core from the sack (if the rng gods favor me at somepoint in this regard).