This is an argument I've been having with myself. I find it boils down to preference.
3 Beasts of Nurgle = 3 Plague Drones with Death heads and Rot Proboscis (points wise)
Beasts are incredible tough to take down, with 4 wounds a piece and It Will Not Die, these guys take great amounts of focus fire to take down, giving the rest of your army a fighting chance to get up the board. 2/3 of the time they have more attacks than plague drones, but this is random every game turn, so can be quite unreliable
. Being 'beasts' they also have move through cover and fleet (re-rolling charge distances FTW) which is incredible useful for an assault unit which should be using cover to get as close to the enemy as possible (shrouded is a godsend...literally). But this is their issue, its getting them into charge range as quickly as possible while taking as little damage as possible. Another major issue with beasts is if they get locked in combat with a walker, that's it. Sure they are tar-pitting it for probably the rest of the game, but that's a lot of points to put into a unit just to tarpit. However, Beasts take up an elite choice, slots which are not used very often, with crushers and fiends suffering from the loss of eternal warrior and flamers now giving the enemy a stacking feel no pain.
Drones are slightly more fragile (compared to Beasts), with 3 wounds and no IWND but as you said, they are more flexible. With rot proboscis they can wound non-vehicles on a 3+ and being armed with plagueswords they can glance a vehicle to death, a very nice feature. Armed with deathheads they can even soften up their foes before charging in, something the beasts cannot. But, their main advantage over beasts is that they are Jetpack cavalry. 12" movement followed by 2D6 thrust movement in the assault phase when not charging something is fantastic for getting up the board quickly. They can even do this after arriving via deepstrike, so you can drop down near the enemy, throw your death heads, and then move into cover so you benefit from your improved cover save, ready for the assault next turn. Hammer of Wrath is also a nice feature for a unit which is effectively going last (I 2 is a killer). Drones are also great at getting your Deepstriking units up the board with less scatter, just pop an icon on them and get your drones up the board ASAP. However, bear in mind that fast attack slots can become crowded very quickly, quite a few good options here.
At the end of the day its your choice. I feel Drones are better if you want an 'in your face' unit while Beasts are slightly more defensive, great at keeping the enemy busy and stopping the nasty units getting too close to your side of the board.