Is RFA.NET API old compare to EMA API? why there are suggestions EMA API is better? In this question, the answer is they are the same.
You can refer to API Concept Guide in section 2.4 Functionality: Which API to Choose? for features' comparison between RFA and EMA.
EMA is currently a strategic product. The new features will added into EMA, not RFA.
On the other hand, RFA is feature complete API. We are still support it but we will not add new features into RFA.
Therefore, if you are developing a new application, we suggest to use EMA instead of RFA. However, if you would like to use .NET, you can use RFA.NET because currently we don't have .NET version for EMA.