The featured crystals have increased chances of landing a 6* (1.5% vs 1%) and of a 5* (17.5% vs 11%). So the chances of pulling at least one 5* or better from 2 featureds is ~35%, while the chances of pulling at least one 5* or better from 3 basics is ~32%. Additionally you have the increased chance within that to pull the featured champion, so assuming that is someone that you want, that's an additional bonus.
In terms of pure value, I think it is mostly a wash. So I would go for the featured if the featured champ is someone I really want, and regular Cavs if it is not.
The featured Cav has 1.5% chance for 6*, 17.5% for 5*, 36% chance for 4*, and 45% chance for 3*. The regular Cav has 1% chance for 6*, 11% chance for 5*, 38% chance for 4*, and 50% chance for 3*. Just to make the numbers nice and round lets say you have 60000 units, so you could buy 300 Cavs or 200 featured Cavs. The regular Cavs will produce, on average, 3 6* champs, 33 5* champs, 114 4* champs, and 150 3* champs. The fCavs will produce 3 6* champs, 35 5* champs, 72 4* champs, and 90 3* champs. The fCavs generate two more 5* champs but 42 fewer 4* champs. That's significant because 42 4* champs generate between 11550 and 23100 5* shards through duplication (assuming every champ dups, and some generate max sig crystals). That's between one and two 5* basic crystals which roughly closes the gap between them.
So it is very close to a tie, and the small difference between them is across 200 or 300 crystals. It is mostly insignificant between 2 and 3 crystals. So the tie breaker is probably whether the featured champ is a champ you really want or is just like any other in the crystals. Alternatively, if opening crystals is something fun for you to do, then of course opening three is better than opening two.
Comments
The featured Cav has 1.5% chance for 6*, 17.5% for 5*, 36% chance for 4*, and 45% chance for 3*. The regular Cav has 1% chance for 6*, 11% chance for 5*, 38% chance for 4*, and 50% chance for 3*. Just to make the numbers nice and round lets say you have 60000 units, so you could buy 300 Cavs or 200 featured Cavs. The regular Cavs will produce, on average, 3 6* champs, 33 5* champs, 114 4* champs, and 150 3* champs. The fCavs will produce 3 6* champs, 35 5* champs, 72 4* champs, and 90 3* champs. The fCavs generate two more 5* champs but 42 fewer 4* champs. That's significant because 42 4* champs generate between 11550 and 23100 5* shards through duplication (assuming every champ dups, and some generate max sig crystals). That's between one and two 5* basic crystals which roughly closes the gap between them.
So it is very close to a tie, and the small difference between them is across 200 or 300 crystals. It is mostly insignificant between 2 and 3 crystals. So the tie breaker is probably whether the featured champ is a champ you really want or is just like any other in the crystals. Alternatively, if opening crystals is something fun for you to do, then of course opening three is better than opening two.