I have a lot of experience with Sierra. I don't with others so my opinion probably doesn't mean much. Regardless, Sierra can pretty much let you accomplish whatever it is you need.
They have a worksheet system for trading that lets you make automated trading strategies from an "Excel Style" setup, which is really nice if you are not that knowledgeable in C++ (also has C++). The downfall to that is it runs slower (backtesting - excel). But still usable, IMO.
As far as indicators, libraries, etc. it has everything. Also very customizable. They have created additions to the software for me personally free of cost.
Probably the best part though is their support. Always get an answer under 24 hours, usually within an hour. Most members are knowledgeable about the product and willing to help. In addition, there are some pretty smart people overall that lurk the forums. Good conversation..
GL