RE: Pellets or loose?
I think pellets work very well, and are very convenient. Downer is, they're very expensive. I use powder only, for last five years.
CVA Buckhorn (I've got one, and an older Staghorn) has 300gr max weight for sabots, you can go heavier with conicals/powerbelts. I think they only recommend 150gr charges in the form of pellets, and only with powerbelts or sabots. I wouldn't use a 300gr sabot w/150gr pellets; 90-100 gr. powder is sufficient, anyhow, and will kill the deer dead.
Do let us know how your Buckhorn shoots.