I still own a 2016 Volt Premier (wife's car) and sold my 2012 Volt when I got the Model 3.
Driving: Model 3 acceleration is, of course, way better. Regen actually feels pretty close, but I think the Volt can do 60kW peak (high speed braking + paddle) and the Model 3 is maybe 47kw max? I never liked the Volt regen paddle since you can't modulate it and I don't miss it at all. Having it all attached to the accelerator peddle makes for much smoother driving once you re-calibrate yourself, but I still wish there was an option for more, maybe 90kW, like the Bolt. Autopilot is awesome as long as your expectations align with its capabilities. The Volt definitely wins on rear-view mirror visibility, but the Model 3 is fine there. They're pretty comparable to me in road noise and ride quality.
Long-distance travel: Supercharging is a compromise when compared to the convenience, speed, and availability of gasoline. I'm okay with the compromise, but I can understand how it's still not for everyone, especially in areas that don't have as much charging as here in CA. On a recent hiking trip into the middle of nowhere, I had to put significantly more thought into the route planning and had to stick with major roads which added even more time on top of the added charging time. To reduce the added time, you want to stay in hotels that have charging (which often cost more) and eat while Supercharging (which are often meh chain restaurants).
Interior: Model 3 is my favorite interior of any car. Driver and passenger space is awesome. The glass roof is great. It's solid and, so far, doesn't rattle.
Storage: Model 3 definitely has more overall storage with the seats folded down, but I miss the versatility of a hatchback.
Infotainment: For my needs, Android Auto in the Volt is the best infotainment system on the market, but I like the Tesla system well enough. It's nice not having to connect my phone to use it, but Tesla has the downside of being tied to AT&T, which doesn't have service near my work. I wish I could choose my data provider, but at least it's free for now. The premium audio system in the Model 3 sounds excellent as well and far exceeds the premium system in the Volt.