The general rule of thumb is you use inverters if your power outages are not too long i.e. less than 24 hours on average. However, if your inverter charging system supports it, you can add more batteries in parallel and even achieve a week of battery uptime - depending on the load you have.
If you have outages frequently exceeding 24 hours then you can supplement the existing inverter system with an autostart generator or replace it with a generator entirely.
Autostart gensets make sense (technical and financial) when you have a huge KVA load along with long outages.
The other advantage with inverters is the immediate power switchover to and fro - (all?) generators don't have this function. That means your IT equipment never reboots unexpectedly.
I'm speaking from experience having installed multiple inverter sites coupled with deep cycle batteries - they run flawlessly for years.
Over time (years) you'll notice the backup time reduces gradually as the batteries age - but by this time they have paid back several times over.