Cadence Question

I was on the trainer last night doing some 10' L4 intervals and was experimenting with different gearing. I find it easier to complete a 10' interval with a bigger gear/ lower cadence and am wondering if I am doing the right thing by shifting down to reach a my targeted wattage. For instance, last night I was doing a 10' @ 300w interval, in my 15 I had to spin at 85 rpm, and in my 13 it was 77 rpm. I find that when I am doing L2- L3 work that my cadence is ~90.

So the question is.....

Do I train my body to handle the fatigue of L4 with a higher gear (85- 95rpm)?

or

Do I train myself to push a bigger gear to produce watts esier (75- 85rpm)?


My goal is to build FTP this winter and try some road racing this spring.

Thanks.