Delta's Comfort+ to Japan is a pretty nice experience, as far as coach seating goes, and free for you to select as Platinum.

In standard Economy for someone with no status, I'd recommend ANA. But I do think the additional legroom of the Economy Comfort seat would give Delta the edge in your case.

One other thing to consider: Most Delta flights to Japan are operated by 767 or A330 aircraft with 2-3-2 or 2-4-2 seating, which is great. The DL 777 is also fine, although with more middle seats (3-3-3).

In comparison, check the exact seat configuration on your ANA flight - they have a million different layouts. Some are great (2-4-2 on the 787, or 2-3-2 on the 767)... some are weird (2-4-3 on the 777), and some are cramped beyond belief (3-3-3 on the 787, 3-4-3 on the 777).

Don't expect an upgrade on DL, though - although you may get offered a buyup opportunity at checkin or get a better seat on an oversold flight, it's a total roll of the dice and not something you can depend on.