Option 1 is how the datacenter at my old job worked, only it was a 460v 3-phase system with a single large UPS (and generator). There are several single points of failure in that system, and we managed to find most of them during the 10-odd lifetime of that datacenter:
- Generator failure. If the mains outage is longer than the UPS can supply load, the room drops hard. Happened (actually, the circuitry that informs the ATS that the generator was online failed).
- Automatic Transfer Switch failure. As above. Happened.
- UPS failure. When power goes out, the room drops hard rather than come back up. Happened two months ago. Nearly stopped my heart. We survived with no equipment failures, though.
- PDU failure. Can manifest as the entire PDU failing or just individual circuits/outlets. We hadn't had this one yet.
Option 2 is where we'd like to get, but as Wfaulk points out, you need to be very careful of loading. The reason for this is because power-supplies can function in two modes:
- Hot-failover. One PS will be drawing 100% of power for the server while the other is idle.
- Load-balanced. The power-supplies will be drawing 50% (or 33%) of power for the server.
What this means is that each power run will have an unpredictable amount of load on it ranging from 0 to 100% of the room's load (or in your case, rack). Each power run needs to be able to supply 100% of load. In the event of a mains outage, unbalanced load will mean that one of your two power paths will fail first (the one with more load, obviously) which will force all the devices to draw 100% from the remaining UPS. This needs to be engineered in from the beginning.
In a rack situation, Option 2 takes more rack-space, but does provide better protection. In my experience, Mains failures tend to take out all of your power circuits not just a single one (barring some unobservant person flipping the wrong breaker back at the panel), so the added protection is small. But running two UPSes able to handle 100% load means that your stuff will stay up longer in the event of a mains failure.