The decision between round and tear drop shaped implants is a personal one.
After mastectomy and tissue expansion reconstruction it is hard to look natural but should that be your goal then tear drop shaped is a better choice.
The benefit of round implants over tear drop shaped is that if you get any rotation of your implant it is less of a concern with round implants than tear dropped. There are also some theories that over time round implants, especially after being subject to the contractions of the pec muscle turn a round implant into a tear-drop shaped implant.
I hope this helps and good luck.