Most likely prefer a smaller school setting for their kids.
A lot of it comes down to simple growth and development being in the Gilbert School district. But I'd say in general Gilbert is still viewed as a more desirable school district by most people. I have had good experiences in the Ames elementary school with my kids, though some are better than others. I have not been impressed with the middle school, but the new Principal they brought back seems to be working hard to improve things. Based on everyone I know in both districts, some that have attended both through moving or open-enrolling, I'd say Gilbert has done a better job in the elementary and middle school levels while Ames has the advantage in HS. Still, probably splitting hairs. If your kid wants to do well, it's going to happen in either school.
Most of the parents I know that were adamant about sending their kids to Gilbert went to small schools. Granted, it's not that small a school by Iowa standards. I also think it's a little deceiving to look at test performance at a school like that, when it's almost exclusively mid- to high-income, has a tiny English as a second language student population.