I have both, I think its pretty necessary for short xc races. Both my bikes take he same race wheels, ie I scrimped on one set of race wheels, and one set of training wheels with cheaper tyres, one stages power meter that goes on both bikes. Three years ago I would use my hardtail for 8 out of 10 races, this year its 2 out of 10 using the hardtail. But on certain courses its defiantly the correct option. So if I had to choose it would be FS.
training is mostly done on the HT, less maintenance, and it can go on the trainer.