I think they're both great, I'd say try both. Get a whole packer brisket too and make your own corned beef with some morton's tender quick too.
The point about pre-cut flats sometimes being the "in between" piece is a GREAT point. Really look at the bag and see if you can see the grain, if you see it changing direction you've got a little of both and I'd put it back.
A lot of people find that pre-cut flats have more fat removed then they'd like.
The point is fattier. Some days I like that, some days I don't. They're both great though.