I have never use a FX-N, but I currently use a Rolleiflex GX, and think it's fantastic.
I have also used a GF670, and that's probably one of the best cameras made in the last 10 years.
For me, the GF670 is easier to use and easier/quicker to take shots with. As you say, you're taking family vacation photos, so you want quick and easy I guess. For me, the GF670 is quicker and easier. I find focusing a range finder a lot easier than a TLR or SLR, but everyone is different. Remember a TLR screen is reversed left to right, and it still sometimes catches me out.
Both are beautiful cameras, but I think in terms of sheer classic design and stunning workmanship, the Rollei wins it.
For walking around, slipping a camera in and out a bag, GF670 wins. For tripod work, I'd probably go Rollei.
I would say out of those two, there are no bad decisions to make, both would be delightful cameras to own.
If think if I've got time on my hands, I'd maybe rather walk around with a Rollei, if I'm slightly hurried around as we often are on vacation, then I'd go GF670. But they are both so lovely, it's like saying if you'd rather have week in Kauai or Maui, both would be great.