If you want to max ROI, then surely all that matters is what value you place on UA miles.

At $5 vgc fee + $1 MO fee and 1.5x on the UA card, you are buying UA miles at <0.68cpm
(Not counting travel/personal time).

If you just do Spark you are making 1% profit on every transaction. Therefore using the UA card instead of Spark also carries a 1cpm opportunity cost.

With these numbers, if you think you can extract more than 1.68cpm redeeming UA miles, then you should just put everything on the UA card and forget Spark.