Serious question about aerobic fitness so HIIT surging or steady state cardio which is better especially for an older person

I hop on my assault air bike for 21 minutes I've tried HIIT steady state and altering my heart rate by surging but not dropping down to zero like HIIT. I feel better when I do 21 minutes but surge in the the session. I cant really get HIIT to work for me and I've heard a lot of bad things about steady state cardio like it slows your metabolism.

SO what the fuck is better?

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