My limited understanding is it isn’t “either” type of training… it’s a mix of “both” (plus more).
For cycling, it looks like some variety in workout intensity is needed to boost performance, while ensuring enough recovery to prevent overtraining and injury.
So a weekly programme might include 1 HIIT session, 1 (or 2) longer rides (2+) hrs, and a recovery ride at Z1/Z2, plus some days off.
Personally, I would put some resistance training in there to build up other muscle groups too. Building/maintaining muscle takes more energy each week than the HIIT session.
I am not an expert at any this, so my $0.02 is worth exactly that. I highly suggest continuing your own research and adding to your personal toolbox before committing to only one type of training.
I’m 53 years old, and have dropped from a “moderately-active” 95kg to an “increasingly active” 89kg, in 6 weeks, by trying to break this smart trainer while it’s in the warrantee period ;-). I’ve lifted weights for 4 decades, although it doesn’t really look like it, LOL.
I firmly believe a variety of workout methods is needed for recovery, adaptation, and motivation.