We KNOW the hydronic will do the job. We SUSPECT the mi i-split will
need to run the electric coils at least part time to do the job if it
is capable of doing the job. I'd put in the hydronics. The mini-split
will still be used for AC - correct???. If so, the mini-split can
provide heat when the basement is a bit cool in the summer
/spring/fall and the hydronic boiler is not fired up.

I was leaning in the same direction.