I don't have a Model 3 yet. So my opinion is only based on what I've read.
I like the phone key as a concept. If it worked reliably, I'd be OK with it most of the time. The biggest problem is the frunk access would require me to get my phone out, unlock it, run the Tesla app, and the unlock the frunk. Not fun if you're walking towards your car with a couple bags of groceries. My understanding is the trunk does have a button to push on the back, so that at least will open if your phone works as a key. But if I had a fob, I would already have my key ring in my hand, be able to find the "frunk" button and push it when I get near the car, to pop the frunk. That said, it's not that often that I would need access to the frunk, so if the phone key worked reliably, I'd be totally fine with it.
However, my wife absolutely does not want a smart phone. She has a regular dumb cell phone, and she only uses it to make calls or send a text message. So for her, she'd have to use the card key. And that's going to be a pain, especially with only the driver's side pillar having the reader. We have 2 kids, and to let our daughter into the backseat, she'd have to unlock from the driver's pillar, walk around to the other side, and let me daughter in, walk back around and put my son in the seat, etc.. Be much nicer if she could just walk to the passenger side and unlock it directly.
But the best would be phone key for me, fob for her. But if I had the option of a fob, I'd just use the fob everytime. It would work the same as the phone, except also give me buttons for lock/unlock, give me access to the frunk, and I could still pair my phone with the car to remotely (via LTE/wifi) do stuff like pre-cool the cabin.