Before I offer my suggested course of action, I need to provide my background. I've been playing since 12/23/16. I'm currently Dangerous/Elite/Ranger with 1.2 billion in assets. I own an almost (lacking armor upgrade) A rated Corvette, a trade conda (waiting to be refitted to mining Anaconda), a trade Cutter (bought 2 weeks ago), an A-rated Python (multi-mission use), an A-rated Clipper, Asp Explorer, A-rated vulture, A-rated Cobra Mk3, A-rated DBS, etc.
I faced the same dilemma a few months ago that your facing now. I wanted a Python, but I loved the clipper; by the time the clipper could be A-rated, I could buy a Python (stripped down for trading). The new AI is something to consider, but my advice will remain mostly the same.
An A-rated clipper is a fun ship to do trading in (though excluded from outposts), in bubble exploring/cruising, and doing missions. Due to the outpost exclusion, planetary bases are your friend, so look for those if your doing missions.
The pitch rate on the clipper is higher (can visually verify now thanks to stats in 2.1) than the Cobra Mk 3 and the Diamond Back Scout. While you will drift and sluggishly change thrust direction, you should almost always be able to keep your enemy in front of you. So you can dish it out in combat. Being on the receiving end? I didn't care for that in 2.0, but managed. I seriously don't know now in 2.1.
What I can tell you is that if you sell your clipper for a low rated python, you will probably hate it. That's what I did. I was so at the sluggish pitch rate of low end python (which I planned to use for trading with outposts to make $$$) that I resold it immediately and went and re-purchased my clipper. After a couple of days of cooling down, I decided the Python wasn't that bad, stripped (but did not sell) the clipper down and re-bought the low end Python. I then did lots of trading until I had enough for a trade conda, and then the money just flowed in. However, all of this was before the (potentially) super deadly AI in 2.1.
If you enjoy doing missions (as I did), which are even more fun now in 2.1, either do them with your clipper or the cobra. You will not want to do them with a Python until you can get it A-rated. At that point, it can take damage/dish it out/and has an Ok pitch rate. As others have suggested, you may want to pick up an Asp Explorer and use it as a mission running ship (good cargo and great jump range potential). The clipper will work too, but you will probably miss out on a lot of missions because they don't have Large landing pads (this drove me crazy).
I am currently doing missions now in my A-rated python, due to large cargo capacity and ability to land at outposts. But when I get attacked, it's messy; I haven't died, but I've had to continually run a lot. If I were flying a beginner python, it would've died every time.
So my advice is: figure out what you want to do and then fly the best ship you can afford for that job. I advise against selling a ship you really like. Just strip it down (sell what you can, downgrade what you can't) and store it later for refitting when you have the $$$. I'd recommend doing this at the stations that have the !5% discount on ships/equipment (Lembava/Goldstein station, Diaguandri/Ray Station, LHS 20/Ohm City (Corvette discounted), Harm/Gentil Hub (Imperial Cutter at !5% off!!!!) Gunapa\Haber Orbital (Clipper/clipper bulkheads 15% off), etc.).
In summation: the clipper is like a luxury sports car that I just couldn't part with. I could make more money doing missions in my Asp, at the time, then trading in the clipper. Doing missions in the clipper was frustrating because of the landing limitations. Combat in the clipper was ok in small doses, but I didn't want to risk it in a RES field; besides, my vulture was far more efficient killing for bounties. Once you own a clipper, it's hard to sit in the cockpit of anything else. You'll need to decide what you want to do and then pick the right ship for the job. I just don't recommend selling any ship from the clipper on up (unless you hate the ship) - just strip it down and use that money to buy what you need; you can rebuild the ship later and won't have lost any money (the 10% loss on the sale).
Sorry if this was a bit long. It's my first post here and I don't post much in general.