As it was, a primary cause of the Gadsden Purchase was to acquire some less mountainous terrain upon which to lay a railroad through the newly-American lands.
However, let's say that one of those Cession-Swap scenarios happens. The US manages to take Baja, and the US loses everything south of the Gila River. Sort of like OTL - only with the Baja California Peninsula. A bit more than OTL, but it's not unreasonably unrealistic.
Would Polk, Gadsden and Co. still go for the lower parts of our timeline's Arizona? Remember, cities like Phoenix had not popped up yet. Tucson existed, but as a small, primarily Mestizo settlement.
So - would the US still make attempts to buy southern Arizona? Would Mexico cede and/or sell it at all in this scenario? Does the US find the railroad important enough to get even more land - and pester Mexico once again?