I said it was "very important", and both have huge story (initial) story ramifications and should be played.
But I also factored in how enjoyable they are, the cost-to-gameplay value, and whether 20 seconds of important story context is worth the asking price.
For as much as I like Arrival and its ramifications on the start of ME3, that "ramification" is immediately negated as soon as the Reapers arrive and that always disappointed me that there was never any larger repercussions of what happens in Arrival (the outcome happens whether you're involved or not).
Leviathan, meanwhile, I do find to be very important to the ending of ME3 and provides much needed clarity. I won't argue that. But it's also arguably the weakest and most uninteresting DLC of ME3 to actually play, outside of the last-minute story revelations. Even "Omega" I found to be a vastly superior experience to actually go through even if it's contained to side-story status with hardly any direct ramifications on the main plot. Leviathan has some good moments, and I enjoyed the underwater atmosphere in the later section, but it's probably the most run-of-the-mill in terms of new areas and execution.
Again, you should still absolutely play it. It's not bad and it's got very important lore tacked on at the end, but I wouldn't rank it alongside the greats like Lair of the Shadow Broker or Citadel.