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Hi Jeff,
I thought I'd wait for the users of the various cameras to comment on their experiences. From our side as manufacturers of the iXon 897 models you have currently, the latest EMCCD models and back-illuminated sCMOS cameras we can make some comments from what we see.

As others have mentioned it comes down to your imaging priorities and optical matching that Gerhard already described. The iXon 897, although now an older EMCCD model should perform very well - that model did have our RealGain and EMCal features that helped avoid issues with EM gain ageing. I would doubt there would be any issues from EM ageing affecting performance, and if it still cools as well as it did then we see these models continue to give comparable performance to when new. The newer Ultra 897 EMCCD models of course have been updated and have revised electronics for lower internal noise, which helps with very low signals and most notably the increased speeds.

The EMCCD cameras still give the best sensitivity so when you need that there is no equal. To maintain similar sensitivity, but improve the field of view an option is the Ultra 888 model (or other EMCCD models that use the larger 1024x1024 sensor), that gives a much larger sensor area the same size as the 4.2 megapixel sCMOS format albeit using a 13 µm pixel size. We find many customers with the older iXon models often go that route over back-illuminated sCMOS when demoing for single molecule experiments due to the hit in sensitivity they would take.

For the back-illuminated sCMOS side, the main sensor generally in this kind of application being the GS400BSI- our camera model with this sensor is the Sona 4.2B-11. If you have enough photons with what you are working with, you can potentially get up to 32 mm field of view (and higher speeds). So it can offer a big benefit for some.

Testing is always recommended against your current cameras if it suits as there are too many subtleties in parameters from labels to optics involved.