Jonah,
It's still early days to answer your question since I have just gotten the ST2, but I will try to help as best I can.
Like you, I did considerable research mostly without the benefit of riding e bikes. With help from reading Court's blog and Currie, they arranged for me to test ride the Peak and the Dash. I didn't like the Peak and the mid drive and the Dash impressed me as quiet and fast but it was and still is a bit large for me in the medium frame. The only dealer within 175 miles was an Izip dealer, so that is what I bought. I raised the handle bars two inches so I would sit more upright which helped a lot with the Dash.
Since I bought the Dash, I have ridden E motion, Pedego, Felt, Stromer, and a few others. I have learned that geometry/fit are obviously very important. Next, for me is a quiet smooth ride and a degree of sophistication with the bike itself. I later learned that range was a key factor as I wanted to take longer rides. I learned from the Dash that I loved being able to go fast! With the Stromer and the Dash, you get up to 28mph+. Once you have it, you will always want it. I like fast cars and now I like fast bikes.
The 2014-2015 Dash is a great bike. If it fits you, there is nothing not to like unless you are on rough roads where maybe bigger tires would be helpful to smooth the ride. After only a few days with the ST2, I can tell you it feels more powerful. The ride, with the body float seat post and about 35psi air in the tires is smooth and the battery life/range is impressive. The bike is a bottom up build that is so clean and simple that it is elegant. The engineering is impressive and the ride is very refined. BUT! If you don't want to spend ST2 kind of money, get a 2015 Dash at close out prices and you will love it. It's a great bike. It even has cruise control!!
You asked the question above...if you had it to do over again, what would you do. If I had it to do over again, understood and could afford the value proposition in e bikes as I do now, and knew what I know now, I would absolutely buy the Stromer St2 over the Dash or anything else available now. That doesn't mean the ST2 is the be all end all, it just means it is a good bike and as you can see from the posts, most are very happy with their purchase.
r