I've played both, Pirates had the discs and the IMDN was a Pro. The theme will be a big influence on you, but game play wise IMDN is in a different league. To me the right hand side of Pirates playfield is dull, the ship mech does nothing for me, the left side is cool.
Also code wise JJP is a gamble IMO, had my Hobbit a year and half and the code is still half baked. That was JJP #2, and this is game #4. Yeah it will get finished, Stern are the same, etc, etc.