I can tell you to go ahead and scratch the GP Slow Poke off your list. I have one and even though it's one of my favorite planes, it has a few habits that will make it a lousy 2nd plane for you. I'd also scratch off the Sig SE. While it's a great flying airplane and you can tame it down quite a bit, it's still too much to handle has a 2nd plane.

When I learned to fly my instructor (and others at my field) to me that I needed to fly my trainer until the covering came off it. I know that I would go out and do nothing but touch-and-gos for a tank or two of fuel. then I would do something else the same way. I wasn't kidding when I said this earlier, when you can fly your trainer around the field inverted then you'll be ready to move up. Like most everything else in life you just need to practice to get better at it. If you try to move up too fast all you're going to do is make a pile of splinters out of whatever plane you get next.