The A321 is really inbetween the 739 and 752 in capabilities.
- It is longer than the 739, but shorter than the 752.
- It has more emergency exits, so it can have more passengers than the 739 (the A321 can carry 199 in a generous 32" pitch, more in charter config) The 752 has a bit more capacity, though.
- It has similiar ranfge to the 739, but much shorter than the 752.
- It can carry more cargo in more standardized containers than the 739, probably similar to the 752.
- It is slightly heavier than the 739, but much lighter than the 752.
IMO Boeing should have put in more exits in the 739, to make it more useful for airlines - then they'd have a much better chance against the A321. As it is right now, both the 737-900 and 737-600 and pretty useless aircraft, and the sales numbers show that.