Either or. Calories burned is a function of the watts you are putting out over the time. Higher watts over a short period can be the same outcome as a longer period with a lower output.
I would work towards the goal that fits within your aspirations. If say you aspire to ride centuries, then work on increasing your output over longer time periods. If say you wanted to have a go at crit racing, then work on putting out more watts over short periods.
Unless you have age or heart/health issues, then 110-120bpm is fairly low. Work towards getting that heart rate up for extended periods. Provided your health, aside from your weight, is good, push your limits to see where the top might be for your heart rate. That can be used for you to put yourself in a harder working mode if it's what you desire, and working in the higher regions on a trainer over winter will mean that cruising along in summer and "smelling the roses" will become that much easier and all the more enjoyable. For example, when I'm fit (not me ATM
) I max out in the mid 180s bpm. I race and I know I can operate in the 160-170bpm range for a good 15min. If I spend much time over 170bpm I know I'll be out the back door faster than last night's vindaloo. I can sit in the 140-160 range for our whole longer 90min rides and when I'm that fit, I can come down from 180bpm to 110bpm in about 2 min on a downhill recovery. I wish I was still that fit, but I'm back to working towards it again which is the main thing