I would (sort of did) choose the A6700 over the A7C and also A7Cii or A7CR. It's a very good camera. If the possibility exists that you could go FF at some point, then maybe some added consideration for the A7CR is appropriate.
I started in aps-c, from dslrs to Sony mirrorless. I skipped over the A7C when adding FF. I also went to the larger body series, A7Riv. The A7C didn't seem to be a great upgrade for me. It added a larger sensor, not a lot more resolution and took ff lenses which, depending on lens selected, can be larger, heavier and more expensive. I do have an A6700, added after the A7Riv and before my A7Rvi.
Although I don't have one, noting that I also went 61mp A7Riv, I'm in the A7RC with 61mp over A7C/A7Cii camp. As noted, plenty of res for crop with aps-c lenses or for increased flexibility and so-called reach for small nature targets or planned cropping.
If your computer system is marginally powerful, you could see somewhat slower processing times. Transfer and storage is more dependent on how much shooting you do. The impact? I'd think 50 61mp files is like 100 33mp files, sort of., for storage and transfer times.
If you didn't like the A6500, I'd strongly suggest hands-on time pre-purchase with any of the A6xxx or A7C series bodies being considered. Not everyone likes them. I started NEX6, then A6400 - the A6700 works for me and I've handled the A7CRs. As I use somewhat larger lenses, not the smallest ff lenses (and the 200-600), for me, I prefer the larger bodies with the increased feature sets over the A7Cs. That's all personal to the different users.
I think the A6700 is very popular with Sony aps-c users. Aside from the AF-On button being a bit hard to find when going between FF and aps-c bodies, I have no real complaints. I notice the difference in resolution say when considering crops in post or if hitting the 100% views but with my viewing and sharing almost all on-line and not "big" prints, it's plenty. The A6700 and 18-135 is nicely smaller than my big cameras and 28-200. My around home uses it seems all I need. For travel to "old cities" and the like, I want wider for either ff or aps-c.
The A6700 might be thought of more like a reduced size aps-c sensored A7Rv not a smaller sensored A7CR. There is a somewhat different feature set for the A6700 compared to the A7Cs. For example, but not only, it has a full mechanical shutter. It's got an eyecup. There are some differences in video features. There are reviews and articles discussing the features in more detail.
I would guess there aren't a lot of A6700s in Buy & Sell as it's got fewer current competitors in Sony land. It's an upgrade in several ways over the A6600 and the others are even less current/capable. Not bad but definite downgrades. The Sigma 20-200 apparently doesn't show often on the B&S forum and it's hugely popular.