You mentioned using your evenings, so I assume you intend to do a degree part time while working?
Right now I'm doing an online masters degree for geotech engineering, 1 class per semester. I took a semester off to study for the PE exam, so I can offer a pretty good comparison between the two. (FWIW, the masters degree is not going to help me all that much in my current engineering job - I'm pursuing it mostly to open up teaching jobs in the next couple years.)
The PE definitely demanded more hours of my time per week. I needed to consciously reserve time on my evenings, weekends, and lunch breaks to study and prepare. In contrast, I can keep on top of my masters work with about 2 hours of lecture-watching per week, plus maybe 3 or 4 more on when I have homework. It's much more manageable, and I can frequently get it all done during lunch breaks alone.
(Of course, you may experience more or less work than this, depending on the program, class, professor, your own study style, etc. My husband is also doing an online masters and spends MAYBE 30 minutes per week. I have another friend doing an online masters and he spends 10+ per week. All in all, though, I think my 2-6 hours per week is pretty typical)
Here's the flipside: I was able to make my own schedule much more when studying for the PE. If I knew I was going to be busy during a week, I could double up before or after to make up for it. With grad classes, you're at the mercy of homework and project deadlines, not to mention your own teacher's preferences for when assignments and lectures are posted. Not a huge deal, but it's certainly less flexible than PE studying.
The other thing? GROUP PROJECTS.
I would rather take the PE again than do another group project. You think I'm joking?
Here's my takeaway: you're already in the habit of reserving large amounts of time during your week for not-so-fun studying, so you're already familiar with the time commitment and discipline it takes. Give or take a couple hours each week, do you think that schedule would be sustainable for the next 4 or 5 years? If so, I'd say go for it.