The difference between Perry's 20% flat tax and Cain's 9-9-9 is that Perry's plan does not include a consumption tax (nat'l sales tax). I haven't been comfortable with the fact that Cain's plan would have even the poorest paying a 9% national sales tax on top of their state sales tax.
My view of Perry is that he would be capable and is genuinely fiscally and socially conservative. He gets a bad rap for some of what he has had to do regarding illegal immigration but unless you live in a border state, you have no idea how tough the battles are. Romney has a similar situation trying to be conservative while governing in a dark blue state like Massachusetts. Sometimes all choices are bad but governors don't have the option of voting "absent" like Obama supposedly did most the time as a senator.
I am somewhat close to the Perry campaign...close enough to probably get a White House invitation if he won but if I had to choose today, I would probably pick Gingrich.