My personal experience was with 2 years in a 2013 Gen1 Volt, so I will preface this with: having driven a relative's 2017 I can say that Chevrolet fixed virtually every little complaint (nearly all related to cabin ergonomics and controls) in the Gen2s, but the cars fundamentally behave similarly so I think my observations wouldn't change that much if I had more experience with a current model.
Fundamentally, they're both really good cars. There are things that Chevrolet nailed in the Volt that the Clarity botches, and a lot of things Honda nailed in the Clarity that the Volt isn't so good at.
I think the biggest difference comes down to size and apparent weight. The Clarity is a much bigger car, for both better and worse. Coming from a Volt, there's way more room in the Clarity, it feels larger and more luxurious inside, it's quieter, and it feels more stable and heavy on rough roads. On the other hand, the Volt is easier to park, and feels much more spritely in all-electric driving--it feels markedly more peppy and responsive.
Which is more important to you will depend entirely on taste.
The other big difference is in the drivetrain; in terms of design decisions made, the Volt feels very much like an electric car that happens to have a long-range gasoline backup, while the Clarity feels like a really good hybrid that doesn't turn the gas engine on much. For example, put the Volt in sport mode and step on the accelerator, and it goes fast all electric. You get very little performance improvement if the gas engine is running, and it won't start unless it absolutely has to. Put the Clarity in Sport and it turns on the gas engine at about halfway down on the pedal, with no indicator of when it's going to do that, and it doesn't get anywhere near full acceleration performance without the gas engine running. It's by no means anemic all-electric, but there is a very noticeable performance advantage to the gas engine running.
Whether you care will depend entirely on taste and driving style.
The one-pedal-driving functionality on the Volt is also much better, if you're into that. Just put it in L and you're almost in one-pedal driving. The Clarity has regen paddles, which are fun, but even at maximum the regen is weak, and you need to put it in Sport mode to get it to hold a regen setting, which has the tradeoff of the car wanting to turn on the ICE when you go even a bit heavy on the accelerator. I've adjusted, but I still find myself wishing for an L gear that doesn't exist.
Whether you care will, again, depend entirely on taste.
The Clarity has much cooler driving technology--mild auto-steer, adaptive cruise control you only get on the technology-package-loaded Volt, and low-speed follow where it will actually stop if the car in front of you stops. Those are all kinds of fun. On the other hand, the basic cruise control on the Volt is much better at holding speed and honestly I prefer everything about it other than the adaptive following. Give me the choice and I'd take the Volt's package--I went from feeling like "This is the best cruise control I have ever used in any vehicle, and it behaves exactly how I wish cruise control would, except for adaptive follow." to "This cruise control is okay, but it's mushy, annoying, and I find myself having to manually manage it all the time."
Once again, which, if either of those two areas you care about, and how much you'll enjoy one or the other or it will bug you, comes entirely down to taste.
The Clarity has no front parking sensors available. This absolutely sucks, and I'm fiercely annoyed by it. On the other hand, it has the right-side camera, which is totally awesome, and the backup camera is much better.
The heater is more efficient in the Clarity when on battery, which is really nice, and makes the lack of a heated steering wheel less annoying. On the other hand, the heated seats take longer to warm up than the Volt.
Those are the particulars that have stuck out the most to me coming from one car to the other.
One more general thing I'll say is that the 2018 Clarity's hybrid drivetrain does feel kind of like a "version 1.0" vehicle; there are glitches, weird oversights, inconsistencies, questionable design decisions, and if you're being picky (emphasis important--not everyone will notice or care) it feels like there are a lot of kinks to work out. The user interface for it is also needlessly complicated and poorly explained (it's entirely unclear that HV Charge mode is for mountain driving, for example). In contrast, even in 2013 the Volt's drivetrain logic felt rock-solid, and I always felt like I knew exactly what it was doing, why it was doing it, and it made sense to me. But then, I'm an engineer, so I'm probably a lot more sensitive to controls weirdness than most people.
The good news: There's no wrong decision. Overall, they're both really good vehicles, and now that there's a mid-sized sedan with an all-EV range of over 40 miles to compete with the Volt there's no reason left not to buy a plug-in hybrid.
