Typically I would trust any Duracell or Energizers that are in date in my STC-9000 and STC-9200. I would imagine the 9100 is similar. My concern is less with what type of battery but when to change it out. Found that out the hard way back in the summer when recording with a low battery.
You have to watch for 'fake' batteries as well. Know where you're getting them from. I generally buy mine from a supplier that deals with TV stations/pro audio (for wireless packs, etc.) because I know what I'm getting is the real deal.
As long as they're genuine, you're good. You can get a "cheap" battery tester at the dollar store that will let you check your 9V batteries. As soon as I start to see a little bit of fade on the 3rd bar (meaning 3/3 - full strength), I change them out. When I used to have a voltmeter (as an audio operator), we'd dump the batteries well before they got to the "yellow part" of the scale; when working in live television, the last thing you'd want is an anchor to lose communication with the director or for their pack to die mid-sentence.
I've always used the same methodology for my battery boxes and preamps -- better to err on the side of caution rather than get burned.