charlottes-p

That varies entirely from person to person. It really depends on how good you are at UT2 and sprint work, and how long you are preparing to spend on each respective session. There will be people who can keep up a very high UT2 pace, but then when it comes to sprinting (and also interval training) can't increase their pace by all that much, relative to others. Depending on how much "all that much" is, it's very possible that they'll be burning more during UT2 fat burning sessions (if we assume you are not stuck to time, and it's a matter of how long you can go on for- you don't do an hour block of interval training, but you would if UT2).

If we look at the sessions being of the same length (sorry, didn't read your post properly before the above), you'll be burning more calories in interval training if your speed/watt output is on average higher than that in fat burning training. Ultimately it depends on how much you slack off during the down-intervals.