You’d be fine with either of them. Do you “need” a full sus bike at Glentress? No. Can a full sus bike at Glentress be fun? Yes, but so can a hardtail.
I suspect that when I take my FS to Glentress I’ll be a bit faster on the descents than when I take my hardtail there. Even on relatively smooth trails I still find it easier to keep up my speed and to keep pedalling when I’m on a FS bike. Then again, absolute speed isn’t the main factor in deciding whether I’ll have fun on a ride, and I enjoy riding my HT at Glentress too.
An RC305 and a Heckler will be equally suitable for Glentress, take whichever one, when all other factors are equal, you prefer riding. If that’s the Heckler then take it and ignore all the “you can’t call yourself a proper mountain biker if you need more than a wonky unicycle with 90psi in its tyre to ride at Glentress” types who seem to validate their existence by being superior on Internet forums. 😉