Sounds like you made a good choice with the 15 II. That is one I don't have. I am using a 21 II, 75E Mini (love, love), and ibrid Nano. I also have a Flex 3401 which I may sell after I use the Mille (on preorder), To me the Flex is a bucket of bolts compared to the Rupes'. Yes, the forced rotation has its advantages, but the greater vibration and walking was a downer. I am very curious to see how the Mille compares.
The question of best beginner's machine has been asked, like maybe every other thread. Here is my take. If budget and level of interest are modest, meaning are you only doing a couple of cars a year or are not into detailing in a big way, the Griots is the best choice. The next step up is the perennial Rupes vs. Flex endless debate. Before I pulled the trigger on the Rupes I scoured the Internet reading forum treads until I was dizzy. There was ONE post that hit a chord with me. He said he favored the Rupes because he could do the roof with one hand on the machine. That did it. He was correct, not that I do roofs with one hand, but you can!
One more point. Having a large (21 II) and small (75E) buffer, to me, is almost a necessity. I am constantly switching back and forth, many times on the same panel. Even if your budget will not allow two Rupes (or even one), I would still have two buffers set up, one for 3" pads and the other for 5" or 6" pads. It is hugely inefficient to have to switch backing plates (assuming the machine allow it). A number of guys use a PC with a 3" plate as a lower cost remedy for that need (essential IMHO!).