The point is that the job, because of its duties, is either exempt or non-exempt from the overtime and minimum wage provisions of the Fair Labor Standards Act.
If you have two people doing the same job and you pay them differently because of the number of hours they work, then it brings into question whether or not the salaried individuals are paid appropriately. That is, it can open you up to a wage and hour claim if they successfully argue that they are not exempt. My advice is to pay them both on a salary basis or pay them both on an hourly basis.
You can make a blanket policy that all part time people are paid on an hourly basis, that's been discussed here before, but I'm not comfortable with that in this situation for a number of reasons. Here are some of the more important ones. First, the hour difference isn't that great. This is not 40 hours versus 20 hours. Second, the part time hours worked standard is so close to 40 that you could very easily end up with some people getting overtime and other people not getting over time even though both groups worked the same number of hours. That sort of schizophrenic approach to compensation will certainly raise an eyebrow as to whether these people are being paid for their time or being paid on a salary basis because of the nature of their work. Clearly some are being paid for the time, attorney for the plaintiff will argue, and that opens the door. Third, different aspects of the nursing profession are currently and have been for some time in the center of debates in terms of both FLSA and NLRA status. In my opinion, you are inviting disaster simply because the profession you are dealing with is regularly scrutinized. Even if the employees themselves are satisfied by the pay plan and would not complain on their own, a third party (such as a labor organization) could get the DOL involved.