What is better viewed on boards, having been a SPO, or a Battalion Commander? Which one is seen as more prepared or qualified?

To become a Battalion Commander you would have had to been a high-performing BCT or CSSB SPO (O-4 position). Once you are a Battalion Commander then your performance in that position will be really what matters to make COL, not SPO time. If you're comparing a SUST BDE SPO (O-5 position) vs. Battalion Commander then it is still hands down the latter.

Sir, I have not been on Officer Promotion Boards, but Leadership Command positions are typically needed to move up.

I don't really understand what decisions this LTC could be thinking about. If he is already an LTC, he either will or won't be selected for battalion command by the command selection board. It's not like he just gets to decide to be one. If he is selected for command and declines it, he won't make COL. And if the SPO is a MAJ position, and he's an LTC what kind of decision point could there be in that regard?