Kindle does landscape now, it was a part of the recent system upgrade.
Format: Not that big of a deal. Kobo can read both but the Mobi support is reportadly bad. There are more stores that sell DRMed e-pub then DRMed Mobi. That said, Amazon's store has pretty much all of the books that you would find at all of the DRMed EPub stores. The long and the short of it is that format really doesn't matter. You can get the same books easily enough in both formats.
Cost might be a factor. I don't know where you live so it is hard to say. I know outside the US the Kobo is less expensive than the Kindle. Inside the US, they are priced the same.