That's a lot fo great points. I appreciate the response. My other lens is 55-250 Canon so I have quite the range covered but the introduction of the new 18-135 kind of throws that around a little bit. I love my Tamron so I'm leaning towards that. The better f stop on it and clarity is also important. I want to shoot video, but I'm going to start getting into vintage MF lenses and think I'll be using those for shooting more. I don;t really want to keep both as I need one to subsidize the purchase of the t4i. I know I can get more money for the Canon as a resale as well. I think this post emphasizes I'm leaning towards selling the STM but just wanted some more input before I commit.
Finances aside, just want to note you do have manual focus in your STM lenses already if you use the back button focus I noted above. It's especially handy for cinema. Since you've already got the STM lens in hand perhaps give it a try?