I’m a big fan of the USB Multitracking on the Mini, as it allows me more freedom during the creative process, by playing/experimenting with each Track much deeper, before comitting to something final.
I’m also enjoying the integrated microphone far more than the radio from the OG. Stereo samples and the extra RAM are nice additions as well.
While i like the formfactor of the Mini, it didn’t really matter to me as much, since portability wasn’t my main focus. Hence i prefer the OG’s Formfactor, eventhough since getting the Mini, the OG has been slightly more neglected 
Which leads me to the most contentious parts between the OG and the Mini. The Pads and the Jogwheel:
I like the way the Jogwheel works on the OG Tracker and i think Polyend did a great job replacing this with the 4 Incremental Buttons on the Mini. They both are a great usability fit for their respective device.
Now the thing i miss the most though, are playable pads for note entry on the Mini (either when simply editing or live recording). This is where the OG Tracker still wins me over. The ability to jam away on the pads or live record is extremely powerful and fast when ideating.
It’s not a huge hindrance in my opinion, but it does force you to approach music-making differently… it’s more conceptual, analytical… vs spontaneous, emotional in my opinion.
Of course that can be easily fixed by using an external controller - i personally use an Arturia Minilab 3 - and there are a couple wishes (1,2,3) for functionality to use some of the buttons on the Mini for note entry. So we shall see…
But for now, the OG Formfactor is still my favorite 