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.
Actually, MO is 70 Cents per 2 VGC's, not $1 for Each (I can buy $999.30 MO, with
2 VGC's).
But be that as it may, the thing is, by using Spark, to offset, there is literally no out of pocket Expense for me to get these miles. Paying nothing is always better than Paying Something. And gas is a non-issue, as I have to be in that town 3 Times Weekly for my wife's medical procedures. If I were to just use United, without the offset, then I would be Paying for the miles. Does that make sense?