They're nothing alike!
Boma is a buffet with lots of choices, some of which are fairly exotic (watermelon rind salad) and some of which are basic (roasted chicken).
'Ohana is served family style. Your table is brought starters (salad, chicken wings, won tons), sides (veggies and noodles), grilled items (turkey, pork, steak and shrimp) and dessert (bread pudding with caramel sauce).
Both are all you care to eat, but beyond that, there's really nothing similar about them.
I second Dreamfinder's advice about not attempting to see the fireworks from 'Ohana. There are a few tables that might have a few, but most do not. The Poly beach on the other hand is quite nice for viewing them.