A lot of people are asking about that
I personally use Sundara more
Alara : larger headband, won't fit small heads. Less tight fit. Thicker sound, more full sound, fuller and more lush voices, great with classical and with being forward in the midrange. smooth treble, thick and enhanced bass
Sundara: More comfortable for most heads, albeit it sits tighter on the head. Lighter. Harder to drive. Thinner sounding. More splashy (a positive thing). Larger soundstage, more instrument separation. Harder to drive. Brighter tonality. More tight and less enhanced bass, I prefer this because it also goes deeper despite being lower in amount.
If both are fed from the same DAC/AMP, or DAP, both are outstanding. If you like a thick midrange, and a lush sound with smooth treble go Alara, if you like a thinner, more neutral sound, with more soundstage and with a more linear response, go Sundara.
You really can't go wrong with either, I have friends who immediately bought Alara after hearing my pair, but I also have friends who have Sundara, both are great headphones, just tuned differently.
Also, both are very reliable