User blog comment:Mittens959/DLC conundrum/@comment-4618888-20120725185036

With Kasumi you get the Locust and a new outfit. And it affects one mission in ME3--it gives you more War Assets than those Bluegear93 mentioned for Arrival. And I personally think the mission is more fun.

Arrival gives you three Research upgrades and a solo mission, and affects the dialogue in ME3 a little bit, plus the aforementioned War Assets.

If you can only get one, I say Kasumi.