Both sons of Elrond got a recent release in ally form, but I believe Elladan to be considerably better than his brother. Each is slightly overcosted without their brother in play, but is often happy to pay an extra resource for a 2 character whilst has an abundance of cheaper options. Without the other brother, neither can really take advantage of their special readying abilities, but once combined they not only improve their main stat but will both ready when an Orc enemy is engaged - Elrohir defending the attack and Elladan helping with the counter strike. The problem is that playing both allies makes this very inconsistent to achieve - which means most of the time you are overpaying for one of the brothers instead of getting an amazing deal.
Luckily, hero versions of both Elladan and Elrohir exist, so it is possible to play one of the allies with the other hero and have a much more consistent boost in their stats - this also finally makes the hero versions of the brothers playable by themselves as once the ally brother is out the hero can also benefit from their stat increases. This is the point at which Elladan outshines Elrohir. To take advantage of ally Elrohir in one deck you need to really be playing hero Elladan. That immediately puts you into a Tactics/Spirit deck - and whilst this is undoubtably a good sphere combination there are superior combat characters in and superior questing characters in and since neither sphere is particularly rich in card draw it makes more sense to have a more consistent deck than to hope that Elrohir appears quickly enough to offset the fact that you are overpaying in Threat for Elladan without his brother. Hero Elrohir is a whole different proposition. Firstly, this puts you in Leadership/Lore which as a combination has strong defensive characters but is lacking in attack and often Willpower - this is where ally Elladan really shines as 3, 2 ally who can often ready for combat is an amazing deal for only 3 resources in this sphere combination - even more so as can make this cost negligible. The other major benefit of this combination is the easy access to strong card draw - Heed the Dream especially - which can help ensure Elladan is on the board within the first few turns and allow the brothers to deal with most combat whilst also pitching in with questing. As an added bonus, Lore also happens to be the ‘Ranger’ sphere so Elladan’s Ranger trait is much more valuable here as it turns on quite a few strong effects.
As a final thought, it seems somewhat odd that the ally versions of these heroes are in totally different spheres from their hero versions. It would have been nice if FF had made the ally version the same sphere as the hero - that way either pairing could be easily put into any deck without automatically making it a dual-sphere deck and would surely have resulted in more people wanting to try out the hero versions of the brothers in their deck.