Demon Front is a greatly misunderstood game by people on this forum, obviously because they think of it only as a Metal Slug clone, when in fact it, its not any more a clone than any other platform shooter. I think they mostly consider it a clone, because the artwork/graphics are on par with MS, but the game mechanics and scoring are completely different. If you play it like it was MS you will be disapointed. If you actually sit down and learn how to chain enemys (does anyone here care about playing for score in MS!?) and learn to unlock the secrets, you'll find its a deeper more enjoyable game.
DDP2 is nothing like DDP, so don't even expect it to be similar (even though the sprites are 90% the same). Think of it more like an omake or little extra that again has different mechanics and scoring. Its a nice diversion, and somewhat short game compared to DDP.