You can do HIIT on the bike or running.
Whatever is most fun! I feel like HIIT isn't very fun haha.
Have you ever tried minimalist running? It's my favorite!
Hey William, what's minimalist running, like running for as little time as possible.. haha
no, seriously, what is it?
Can I ask you a question before I reply to yours?
What sports do you actually enjoy? Sod the ‘calorie burning, don’t worry about being lean/fit’ dare I suggest you are missing the point. Focus on experimenting with exercises and sports, in order to find the one you love to do. Do this, and you should remain lean and fit as an occurrence, as being fit and lean is a symptom of doing the sports you love, rather than the reason for doing sports.
Therefore, as William said, the best exercise is the one you enjoy the most, the one therefore that you are likely to do the most.
However, I wouldn’t choose one over another, if you like more than one, just do as many as you can, this way they remain exciting.
If you have to choose one, so that you are regularly doing it, in order to become proficient, achieve some goals maybe, set a routine that becomes a habit, then the one you enjoy the most is the one to do for sure. Sport and/or exercise, fundamentally MUST be fun, enjoyable, otherwise it is not sustainable and becomes a chore rather than pleasure.
What HIIT are you doing? As stated already, HIIT can be via a bike, running or weights/cross fit. HIIT via a bike or running is aimed to improve your biking or running prowess and therefore has a reason other than ‘leanness’ and therefore I feel would be more rewarding.