Just to add a bit of balance. The campsite at Glenmore is run by FC, is quite large and probably aimed at the caravan / motorhome brigade. We stayed a couple of nights last year and whilst it was expensive, the toilet facilities were excellent. At Glenmore you also have the option of being in the trees or a bit more out in the open, which may be important to you if you land in the middle of midge season. There is also a kids playground just off the site, the nasty Glenmore shop with the dry ski slope (wtf is that about?) and the FC visitor centre with reasonable-ish cafe.
The Rothiemurchus site is completely wooded and much smaller in comparison. Better suited to tents – I’m not even sure you can take a caravan or camper. There is no shop and the toilet facilities are not as clean as at Glenmore IME. The Rothiemurchus site is nearer to Aviemore though and more cycleable than Glenmore – which can be a bit of a drag back up the hill from Aviemore.
There are trails right from both sites.
If you like it more basic / rustic / nearer to Aviemore then go for Rothiemurchus, if you want a shop, playground and cafe virtually on site and 4* toilet facilities and don’t mind the longer trek into Aviemore, stay at Glenmore.