If you're on the highway all the time, the hybrid system doesn't play any significant role. If anything, the aerodynamic body of the Prius (c or otherwise) helps with fuel economy way more than the battery. So, you're paying extra to have an elaborate hybrid system that serves you little benefit in terms of fuel economy. You're probably better off with a small and/or aerodynamic diesel.
Mind you you need the battery to get on the highway, every time there is a slowdown on the highway and to get from the highway to your destination, so it is a benefit everyday.
If you're comparing the c with the liftback based on fuel economy alone, you're still probably better off with the c. Unless you find a better deal on the liftback than on the c and/or if you drive 90% on the highway, the ~$5,000 more you'll pay for a ~2-4 mpg gain (from ~46 to ~49 mpg) in the liftback will take a looooong time to pay back. I'm too lazy to do the math, but at 30,000 miles, $3.80/gallon gas, 46 mpg in the c and 48 mpg in the liftback, the liftback would onjly save you $100 in gas. Oh, and heater usage won't affect your mpg on the highway, since the ICE will be running 99.5% of the time anyway. AC usage will, but not by much. As for the question of diesel or hybrid, the main thing I don't know is the maintenance costs: which parts are more/less likely to break, how much it'll cost to fix them, etc. It also depends on diesel prices in your area. I've seen some places where it's been $0.60/gallon more than regular gas, and some where it's been $0.05 cheaper. If it costs more than regular gas, that'll eat into your savings over buying a hybrid. I'm also not sure how initial purchase prices compare. If you plan on buying a hybrid and keeping it forever, you'll probably eventually need to replace the traction battery - it's under warranty for 8 years/100,000 miles (or 10 years/150,000 miles if you're in a CARB state). This will cost a significant amount if you have Toyota do it, but by then my guess is it'll be under $1,000 for a third-party to do it, and even less if you get a battery from a wrecked c. Though the vast majority of all Priuses ever sold are still on their original traction batteries, and most of the ones that aren't are the first generation of Priuses that are now 10-12 years old. The hybrid components aren't inactive while cruising - take a look at your energy monitor screen when you're on the highway. The electric motor still assists when going up slight inclines and regens when going down. I think this is because the ICE tries to keep itself at its most efficient "gear" ratio (even though it's a CVT). If you're going down steep enough, fuel injection will be cut off (though some other new non-hybrids also do this).
Diesel is the obvious choice for all highway, uncongested driving because of good torque at very low rpms. Yet I still lean toward the Prius family because if gas price spikes you need only reduce speed to improve economy, and because maintenance tends to be less costly for a Prius than for a tdi.
I do a lot of stop and go driving on the freeways where I live. I hardly ever can maintain speeds above 60mph unless I leave super early in the morning or late evenings. The Prius or any hybrid for that matter will do better than a diesel powered car in stop/go traffic where speeds are less than 60. If a diesel powered car was able to maintain speeds above 70mph for long distances the overall mpg "might" be better than an equivalent size hybrid, depending on your destination or distance. For example making a round trip on I-5 from Sacramento to Bakersfield (relatively flat highway) on a daily basis would probably beat out the Prius hybrid if you were going about 75mph or higher. Which is about the typical speed drivers cruise on this stretch of highway. In the end it depends on how you are going to drive the car, for me the Prius C is my daily driver to work and home only. If I lived out in the boon docks and had a longer commute where speeds of traffic are higher, then a diesel powered car might be better, again by a very small margin.
I just replaced my wife's VW Jetta TDI {2010} with a 2013 Prius C. Not only am I getting better MPG at 52 on th3 interstate in WI but what I looked at most is the maint. cost. The maint on our VW Jetta was high especially with out DSG transmission. We drive about 45,000 miles per year. between tire replacement the 79.00 oil changes every 10,000 miles, 425.00 DSG service every 40,000 miles and the 1,200.00 dollar timingbelt change at 120,000.00 miles. We are saving thousands of dollars in just scheduled PM's. Plus, gas is close to 50 cents a gallon cheaper right now. My wife now drives the Prius C and I drive the VW as she drives twice as much as I do. We are saving a lot of money.
A Cruze diesel equipped similar to my Prius C 2 is $25885 + tax, title etc... Yes its bigger and has leather but I would rather save $8k and own a Toyota than a Chevy.
I think you should look at VW TDI as an option, the fuel cost more. but if you will be driving about 80 mph for long distances the turbo diesel can be pretty good. also look into the plugin prius, and volt. if you usually drive 40 miles and then can plug in and work all day. then drive home. the plug in/hybrid may be great for you.
I only have 3000 miles on my PC2 but did drive to Florida and back (1k miles). Gets 55-58 mpg going 60 mph. Gets 46-47 mpg going 70-75 mph. Diesels cost too much, $6000 premium for a VW and the gas costs more, plus VW is unreliable and diesel costs more to maintain