I too wonder why SPXL over UPRO. The volume on UPRO is way better than SPXL. That should reduce spread. But what about cost? I don’t know.
I prefer the UPRO index over TQQQ as SP500 is more diversify. I hate to risk capital on so few companies that occupy QQQ. QQQ is 70% made from AMZN, GOOG, APPL, and Facebook (is that really a company) since it is market cap weighted index. I really like the idea of QQQE. I do not know of a 3X version of that index. But I like the idea of back testing even more. LogicInvest does those things. And they do it very well.