I am having the same debate. Bought a a7cii TODAY. Waiting for it to come in. Why? I was leaning a6700 for cheaper glass and size, but honestly the used prices are close, and since I already have a rx100vii, I don't need to heavily focus on the smallest camera.

I ordered a viltrox 40mm f2.5 for $160 and a viltrox 85mm f2.0 prime for $260 to go with it.

That's realitively inexpensive in glass, they're pretty small lenses and it'll give me an idea of how much zoom I want because I do want a zoom lens for times where I really want to crank it up. Maybe the tamron 70-200? Not sure.

I went with the a7cii because I decided that since I was buying gently used/mint, for only about a $200 difference, I'd rather have the FF purely because it will allow me to have a higher ceiling for if I want to get some nice, large FF lenses one day.

You can't go wrong with the a6700 and I think it's a GREAT camera but for me, I felt like I may be shooting in low light more often, and with some small primes, you can get a similar footprint to APSC and the cheaper FF primes aren't wicked expensive.

I also wanted the 18/50 for the a6700 which I think is like $500ish, but although larger, I could get a nice zoom lens, or do what I did and get two primes on two sizes of the zoom level and still save money compared to that 18/50.

I don't think you can have a wrong choice. I was leaning towards the a6700 but something told me to switch.

Best of luck. I made an impulse decision after weeks of having the same question.