Fer-de-Lance:
The most important advantage is speed and the FdL is fast (around 410), it can run away or catch like 80% of all existing ships, a FdL is a serious threat to every ship, but you sacrifice so much for that: no cargo, power is limited, bad jump range (13-14,5), and the Huge hardpoint is not always useful
but i always recommend the same for anyone who plays in open, "speed", it's your best friend, it will keep you safe, it will let you dictate the fight, and thats why after all i like this ship, it's very good for combat, good fire power (choose wisely your weapons), good acceleration, good agility, incredibly tanky as hell even without shield >< so yea as a final note i'd say that if you are a violent pilot who only wants to kill and destroy, this ship might be what you want
Python:
The very badass Python is a monster, you have lots of everything, cargo, fire power, shield, jump range, medium pad, everything except... speed sadly, in open 80% of all existing ships can run away from you, and if one day you ever loose a fight you likely won't be able to boost away (but should have the time to jump in most cases), if you can pay the bill and don't mind to be a bit slower than the average, this ship is for you
All in all, they will both make you feel like the Terminator, not many people will be able to fight back, no matter which you choose, but if i had to , i'd say the safest and best for combat is the FdL again especially for the speed combined with its natural power so for full time open hardcore pvp it's the best, on the other hand the Python while still powerful for combat is also one of the most balanced ship overall which is a serious advantage if you like to diversify your activities without being forced to change your ship.