I have both. I've been using my MXT Pro for a year and it's a great machine. I've only had my AT Pro for less than a month and it is now my favorite detector (taking the place of the MXTP).
I have no doubt that the MXTP is the deeper of the two. It's also better balanced and it's build screams quality. The coil plug is easy to manage as is the mic plug as you already know. The O'ring plugs on the ATP are a bit awkward to seat and their location is more than a little stupid! The MXTP's chattiness can be annoying and it's pin point scream painful. I can't believe Whites hasn't put a vol control it!
I find the ATP easier to listen to by far even when running zero disc and actually prefer running that way. No way I could listen to the MXTP w/zero disc!
Swapping out the stock coil for the 5x8 on the ATP changed the whole demeanor of the ATP. Lightens up the front end and balances it out really nice. Also the added separation of the 5x8 is pretty incredible with minimal depth loss. If you get the ATP, get the 5x8. I haven't put the stock one back on since!
The tone roll on the ATP is also pretty amazing. If you dig bottle caps, it's on you, not the machine. I found the MXTP to be a bottle cap magnet. Iron audio on the Garrett is another great feature. there are some really good vids out there showing and explaining it. I love it!
Nickel Nickels & more Nickels...the ATP is just tuned into em WAY better than the MXTP. I've dug more nickels in the last month with the ATP than I've ever dug with the MXTP. The upside to that, it's tuned into those gold rings and other jewelry pieces. The down side, it sounds off well on certain foils and aluminum trash. I find the MXTP to be better on beaver tail pull tabs.
I think they are both great but I think the ATP's audio is much better than the MXTP's and it makes it easier on the user to decipher.
Here is my ATP transformation...
http://www.treasurenet.com/forums/garrett/356743-patiently-awaiting-new-pro.html
and this...
http://www.treasurenet.com/forums/garrett/359921-my-new-favorite-detector.html
Plus it waterproof! You don't have to run for cover when it starts to rain.
Get one, you won't regret it. And if you do, PM me I'm sure we could come to some mutual trade/agreement. I wouldn't mind having two!