Both of these tangs are Ich magnets, especially the White Cheek tang, and both need to be heavily QT before being placed into a tank. That should be your major concern before deciding if you should have either one or both in the same tank.
I would get the powder blue, it's easier to care for. Marrone is right though, definitely quarantine each tang. If the tang is very small then a 30 gallon tank should be fine, just add some LR in there, add an ammonia source and make sure that there is no ammonia and nitrites for 2-3 and then get the fish. Do weekly water changes in the 30 gallon and quarantine for 3-4 weeks.
Where u getting the white cheek really nice fish! If u have exp me personally id go with the white cheek it's not as common as powder blue def a beauty