My 2 cents
New Clarity owner, but tested Chevrolet Volt 2018, Prius Prime 2018, and of course Clarity PHEV (Actually tested Touring, but bought a base model) before deciding on the Clarity.
Volt
Pros - highest EV only mileage at 54 miles (EPA rating), more than double Prime and ~10 more over Clarity.
Supposedly, (gas) engine never turns on as long as your battery has one drop of energy.
Con - Lowest ceiling of the 3 (literally hit side of my head the two times I entered them - once on the floor sample, and on the one I actually drove) and I'm "only" 5'9" though my gut probably didn't help

Never got the chance to drive it on gas (afaik), but with the electric motor propelling the car, there is a Definite drag feel to the car; as I said to the dealer guy, felt like I just got back from Costco with 3 months worth of supplies kind of feel. This was present from the first feet up forward to when I was going 95 mph. This was the first electric car I drove, so I thought it "was normal" and the handling is otherwise quite remarkable (see 95 mph). This was more noticeable when on the On ramp (literal inclined ramp) going on the freeway.
In the middle of the back seat floor where your legs would be if you sat 5 people in (this care is rated 5 seater with seat belts for 5) is a fairly high "hump" where part of the battery is. As in any one sitting there better have short legs (as in kid on a car seat legs), or knows how to man spread (even if they're female). I thought this was exaggerated in the pictures, but when I saw RL, it's almost as high as the seat itself
Most expensive of the three, at about 3K more on base model to base model with Clarity (and ~7K vs Prime)
Prius Prime
Highest HV mpg at 54 (vs ~44 for both Clarity and Volt) for those times when you really want a Hybrid and not a plug in. Not really a pro or a con, IMO.
Pro- cheapest of the bunch. You can get the mid level for about $26K (Base Clarity at 31K, and Volt at about 34K)
Highest ceiling of the 3. Leg room could use some help but didn't feel claustrophobic at all
Weird thing; per the dealer rep that took me on my test drive, they don't normally charge new cars, so I ended up driving it as a Hybrid car (vis a vis EV mode). Can't really compare after that
Clarity - reasons why I went for it (not necessarily in order of importance)
Roomiest of them all. Ceiling not as high as Prime, but more legroom over all
There's still that hump like the Volt but not as pronounced. I probably still won't let 5 Adults seat in the car though
Higher than Prime's EV only miles, and more than enough to cover my daily commute (primary reason for getting a Plug in, as opposed to a Hybrid) without using gas. Dealer did warn me that Gas engine Could turn on even on EV mode (as opposed to Volt's) if I'm too heavy on the gas. But other wise, still better than Prime's 25 miles EV and without the Volt's draggy feeling (that I noticed was missing on both Clarity and Prime) And I don't hit my head

Mid priced compared to the other two; did not even consider Touring once I found out what exactly my additional $3500 would get me
Looks. Eye of the Beholder and all, but both Prime and Volt are considered hatchbacks, while Clarity is a sedan and the looks matches those names.