The 5516X would be the better choice as long as you don't need an expansion I/O card for fiber-based connections.
It has better throughput and even if you should decide to load up all the FirePOWER services (IPS, URL Filtering and Malware protection) you could fill the 100 Mbps pipe before it ran out of power.
You won't get Antivirus like Trendmicro on any of the current ASAs though. That never really caught on among customers and has been discontinued altogether. That said, the FirePOWER services protect against typical modern day threats much better. AMP for Endpoints is one thing to think of for them.