With the length of JBL's career and his competition during his prime, right this second I would wanna lean to him.
But the more I consider this topic, and truly think about what the Miz has done, I am going to say him. Sure, his first few years were rough and a lot of it due to JBL's own doing. JBL's first few years were nothing special either. Justin Hawk Bradshaw never did shit. Also, he was a part of The New Blackjacks and that did nothing either. It wasn't until he took a darker role and joined with Undertaker's Ministry Of Darkness, and Farooq that he finally became a little bit relevant. Even then though, The Acolytes were sort of an after thought compared to The Hardy Boys, The Dudleys and E&C.
Over this last year, Miz has rose to a level of stardom that I am not sure JBL even reached when he was WWE Champion.
JBL won the Tag Titles numerous times, when the tag division was hot. So, he definitely deserves some credit there. But Miz won the tag titles with a couple different partners over the last year or 2 and won the US Title. Then he wins MITB and captures the WWE Title.
The Miz definitely has created a massive buzz by becoming a Reality Star turned WWE Champion. More buzz I think than JBL had ever been a part of.
People look back now and give JBL a little more credit then he deserves. When he WAS Champion, everyone called him a Tag Team wrestler or Mid Card guy that was WWE Champion.
He received some of the same criticism that Miz gets now.
So, taking everything in consideration, I will go with Miz, because between him and JBL, JBL was okay, but The Miz is AWESOME!!!