I say 739ER.
Will whip a 321 9/10 times
Plus more flexibility with the 737 NG family.
The NG family yields no more flexibility than the entire A320 family does, not to mention the longevity of the A320 commonality allows commonality for about 4 decades of possible game-time a massive
plus. The only thing the NG family gets you that the A320 family doesn't is a range over 3000NM, which isn't that big of a plus anyway on narrow-bodies.
The 739ER requires the big MTOW variant to be remotely comparable to the A321 and with that upgrade it costs considerably more, to the tune of about $10M. And the 739ER requires another cabin crew, increasing your crew costs by 25%. And it seats about 5% fewer passengers.
Yes, the 739ER is superior in fuel, but considering the added labor and upfront costs and lower ticket revenue, it isn't that much of a difference. At best, they/re dead-even competitors, the 739ER is definitely not better "9/10 times". If ever there was
something that the 739ER option was better at, it would be the sub-1000NM range that the OP is talking about, as that only requires the base model which saves a great deal of money, even so the difference is minimal at best.