what Dusty just said, but also consider both of your suggested locations are after joints in the pipework. In my experience there's a big potential for measurement errors unless the sealing at the pipe joints is perfect.
I mounted sensors in positions close to those used by KTM on the OE pipes when I installed my LM2 on a 950SM.
If you already have lambda sensors, why not disable them and use the OE mountings? Or if you want to maintain closed-loop operation, some of the wideband kits have the capability of emulating a narrowband sensor which can be used directly by the ECU... and you'll still get your wideband data for logging and export as you wish.
24" mounting from the exhaust port is not so easy on a bike. I've mounted many much much closer than this with very few problems. Providing there's a reasonable airflow round the sensor to keep it cool, everything should be good.