If I'm reading your post correctly, one job is full time and reasonably well-paid, while the other is PRN and extremely underpaid. Even leaving aside the issues related to specialty, that seems like it should be a big red flag re:the second job. It's very unusual to hire new grads to either float pool or PRN jobs, let alone both. New Grads need an extended preceptorship (several months at full time hours) to function comfortably on their own in a given specialty, and take a year or more before they reach competency. How will a PRN float job provide that kind of support?
You say they've had good luck with new grads in the PRN float position before, but have you met these happy, now-experienced employees or is that something a hiring manager told you? It's not impossible, but it does sound implausible. It may be that they're willing to hire new grads simply because many new grads are willing to work at bargain-basement prices and are less equipped to question working conditions, and because it's no skin off their backs if the new grads burn out or risk their licenses.
I say this because I was offered a PRN job as a near-new grad (had already been hired in a hospital job, and was out of orientation, but barely) by an agency which reassured me that I would be a perfect fit. It seemed like a great way to gain some additional experience, but I ultimately decided against it. With the benefit of hindsight and a few more years of experience, I'm SO glad I didn't take it and risk my license or my patients for an unscrupulous agency- they wanted me not because sending brand new nurses in to a highly independent job is safe or effective (it's not), but because I was too green to know better, and they couldn't hire more experienced nurses because of low pay and poor working conditions. Sadly, I think this is pretty common in nursing. Always remember that your license, your health, and that of your patients is more important than any individual job opportunity- no matter how hard a smiling HR director tries to make you feel otherwise.
Take the job with reasonable pay, benefits, hours, and support, in a field you're passionate about. If you become an FNP you'll have plenty of time in the future to work in other settings.