I loved my Cobra Mk III, and stayed in it until I could afford a ASP Explorer (with a few upgrades). I wouldn't sell the Cobra though. I've kept all of my ships that I've flown. Spending so much time in them, you tend to bond with them, and sometimes it is nice to go back and fly them, and sometimes there are missions that are better accomplished in certain ships.
My rule of thumb: fly small and agile when you can, but if you need to carry a certain amount of cargo, carry a certain number of passengers, travel a certain distance, land on a limited size pad, or need a certain amount of armor and weapons, fly the ship that has the best chance of completing the mission.
Yes, the Python is a great ship. It is like the Cobra's very big brother. I use my Python for carrying as much cargo as possible, while still being able to land at outposts (something the big three can't do). But the Python is an expensive ship, and it will take you a very long time to get one (with the upgrades you should have) with a Cobra Mk III. The ASP Explorer is much easier for you to afford, and you can easily earn hundreds of millions if you play the game right.
The Anaconda has some nice features, but it isn't for everyone. There are other nice big ships that have their own benefits.