Which scenario would be worse?

A chevy impala t-boning a two door honda civic at 60mph or a semi truck rear ending an SUV at 15 mph?

The T-bone is worse
60mph is faster than 15mph AND its to the middle of the car, where there's no protection. Unlike the bumber at the back which somewhat protects against a rear ending

What is the formula for energy? Mass x velocity?

Neither one is going to end well. But, my bet would be that the T-bone would run a much higher risk of fatality. The truck may hit harder, but due to the movement, forward to back, there's more clearance. Whiplash or a broken neck is possible, but unless it pushes the vehicle into something else, your risk of fatality should be less. The hit from the side, though, could send you into the side window. If it impacts on the driver side, where there's less protection, you could die.

I was struck from behind by a truck, not that large, at 25 mph and came out fine. I was at a stand still (traffic light), and was checking out the women on the sidewalk, in neutral.