I'm not sure exactly how useful dual cores are for phones, as, I'm not sure if apps are coded to utilize multiple cores. It will probably make things faster though as multiple apps running simultaneously could take advantage of separate cores. So, it's probably a good thing.
Regarding the Thunderbolt and Charge. I was looking for a 4.3 inch phone and picked my X2 over the Thunderbolt and Charge. Heard bad things about the Thunderbolts battery life, and, the Charge felt like a cheap plastic toy with a horrendously garish screen. I wasn't too concerned about 4G at this point, so, I went with the X2. It seemed like the most solid of the three, the display was the nicest, and, it's just an all around good phone. My buddy has an original X, so, I got to spend sometime using it, which, helped in my decision.
That doesn't really help you though I guess as it's not 4G.
Since I bought my X2, I haven't kept up on phones like I was when I was getting ready to purchase a phone, but, of the three you listed, I'd pick the Bionic. Mainly because I didn't like the other two. If there is something coming down the line that's better than the Bionic, I can't speak to it. Maybe someone else will have some say on that. But of what you listed, I'd go with the Bionic. I'd still do some research on what phones are coming out. A good site to follow is
http://www.bgr.com/.
Also, I just made the vote on the poll a four way tie. Doh!!