The Iowas would probably be around longer, though the proper response here would have to be the Marines having the budget to operate them themselves, using the same sailors that operated the Navy's amphibious ships. If the goal is keep up with the Carriers, the Iowas get the job, because of their additional speed. The Marines, however, would probably want the Montanas, for extra firepower - the fact that they could only make 27 knots in that regard is irrelevant, because very few amphibious ships could make that - the Iwo Jima and Wasp class ships were only capable of 22 knots, the Tarawas 24 knots, the Austin class docks 21 and the Newport class tank haulers 27.
What happens largely depends on budget. If you want to keep battleships around in a limited role and have the dollars to do so, work on improving their ranges and have the Marines run them, as well as knocking the crew size down. The Iowas were proposed for additional refits in the 1990s, which would have knocked the crew requirements down to about 900. The Montanas will require more, but if the Marines are thinking this way, getting it down to 1100ish would provide better bang for the buck.
If you have to keep one, keep the Iowas. But if you are looking at optimal world, what might work is to have two Montanas keep in the fleet, operated and funded mostly by the Marines, while the Iowas get the proposal proposed for the Kentucky to make them into missile battleships. All of this costs a very large amount of money, of course, which is the primary problem with the battleships in general.