I use Duracell 2650mAh rechargeables in my Pro-97 (20-527). They seem to last forever. (At 2650mAh I would hope so) I don't think I've ever used the radio for enough continuous hours off of AC for them to drain, as I remember to plug it in. I use the same in a Grundig G5 portable shortwave radio that also charges the batteries. I don't use a separate charger though as both radios can charge.
I previously used some 1800mAh generic batteries, but I believe those would drain after some heavy use and not plugging in the radio to top them off. I think they got pretty old too.
Ultimately it comes down to usage and your battery power.
- Pro-97 Current Drain (Squelched) 90mA as per the manual, but I figure/guess 100mAh since the radio isn't always squelched, fortunately, and the issue of nominal power needs, where rechargeable don't fare as well near the end of the cycle.