I'm going to give a disclaimer first: I have not installed baseplates on a Mini myself nor do I have any direct knowledge of comparing the inconspicuousness of the Blue Ox vs Roadmaster. I'm only going to make comments because I once considered a Mini for a toad but never actually did so my comments will not mean much or give any meaningful help to you so I'm just kind of filling space until somebody who owns a Mini chimes in.
First, folks on this forum know that I always try to endorse forum sponsors and recommend their products when applicable. However, even though Blue Ox is a forum sponsor, I've always used Roadmaster products on all my toads and the several tow bars I've used all have been Roadmaster.
But in this case, I'm not really endorsing one over the other. It sounds as if you've done enough research to have already viewed what I'm going to point out but just in case you haven't, I'll go ahead and give you the information nonetheless.
It also sounds as if you are going to do the installation yourself. I've never done an installation myself but have helped another RVer install Roadmaster baseplates on his vehicle. Even though I personally wouldn't attempt it myself (I just don't have the confidence), it was a lot easier than I had anticipated.
Again, I have to believe you've already went over these but just in case, here are the installation instructions for both in a PDF:
Blue Ox BX 1307
As far as inconspicuousness, it looks to me that they're both good at doing that. Roadmaster may have a slight advantage, IMO only.