This has been raised time and time again so I can give you all the info in a handy snapshot.
- Both servers have an almost equal number of players and activity
- PvP on both is pretty active. Overall impression is that Sirens on Union is a bit busier than Defiant whereas Defiant may be a touch busier in Warburg
- Only other difference is that Union is the unofficial RP server. That doesn't mean everyone on Union RP's all the time(quite the opposite), just that RP is a little easier to find on Union.
Other than those very minute differences, there's not much to choose between them.
There are millions more people on Union than Defiant, oh wait, that's Vigilance 
Wich server has the most people, Union or defiant? only have toons on Defiant right now, but often lack some people. Are there more on Union? And is there more pvp on one server from the other?