I own both and have engineered up both of them quite extensively. I've spent the last few nights swapping between the two going to a haz res and laying waste to any pirates.
Here's my approximate python build:
https://eddp.co/u/1eCN97Zr (obviously the values are off a bit given the new engineering, but it gives you an idea)
Some general Python Pros:
-More internals so you can fit a larger collector limpet for mats gathering after bad guys go boom and still have good hull/module integrity
-More cargo
-Higher shield capacity
The main negative is slower top speed/boost speed. Both can have similar jump ranges.
IMO, the Python is a better "all around" ship for bounty hunting, mission running, planetary prospecting.
The Chieftain really shines at combat. It's quicker and more maneuverable than the Python so where it lacks with less shielding, it's better at avoiding being hit in the first place. The hardpoint placements are great on it, and the cockpit view helps when dealing with pesky smaller ships. Since I do most of my ship 'building' in coriolis, I don't have a link for it, but I've got 2 long range pulse lasers and the rest of the hardpoints are overcharged MC's with a mix of the standard effects (incendiary, corrosive, etc).
At the end of the day, I'll always love my Python as I see it as a very capable ship suited for doing a variety of tasks. But, after spending time with the Chieftain in a Haz Res, I feel that it's a better at combat than the Python, though not by a significantly large margin.