Marissa Gonzales

I totally understand your dilemma. When I got my Master's degree I chose to stay at my undergrad institution instead of going somewhere else and I still wonder what kind of opportunities I missed out on because I didn't take a risk and go to a different university. That being said, I don't think I was being a coward either and I had plenty of success with my MA. I had a good relationship with the faculty at my undergrad institution, an advisor in my areas of interest willing to work with me, an established network, and all the benefits of being in a small department.
I found your post because I've also been accepted to the UCI Informatics program, which was/is my first choice for the PhD. From my POV this is not a bad option for you because 1) you're familiar with the department and presumably already have relationships with the faculty, 2) the faculty there share your areas of interest, and 3) they fully fund their students. But you might find something incredible at GaTech that you want to pursue.
It sounds like you and your partner have a plan for either situation, so my advice to you is this. The deadline to accept the offer at UCI is April 15th and I assume that GaTech has a similar timeline. Go to the campus visit at GaTech and go to the one at UCI. Find out what both programs have to offer you in terms of potential projects and see if you "click" with any of the faculty. Sure you know the faculty at UCI, but from my experience faculty members interact with their graduate students differently than they do their undergraduate students so you may see them in a new light. I'd also make sure that GaTech fully funds their PhD students. From my POV, taking on something like a PhD that isn't funded for the duration seems like a bad idea. And if you have the time I suggest exploring Atlanta a little bit and see if you might like living there. Not only would you be committing to GaTech for 5+ years, you'd be committing to living in the ATL.

I hope any of this is helpful to you.