I've only used the HD438s, but from reading reviews (which I assume you have done),
the HD215 doesn't have a lot of bass. The HD 215 is more of a block-all-outside-sound type
The HD438s have good bass but it's not so overpowering that the bassline is the only part of
the song that you can make out. If you want really bassy headphones, try the Sony XB series.
HD438s are comfortable for prolonged usage, that's for sure.
The HD515 is an open back headphone while the HD438s are closed back, so the bass on
the HD515s and the HD438s should sound different. Apparently, the HD515s have epic bass
extension, which means you'll be able to hear the really low bass tones. I would go for the HD515
just because it's more expensive and has better technical specs.