My advice is to use loose powder. It's cheaper to shoot and it allows you to customize your powder loads down to 5gr increments...........pellets can't do that.

Also, just a word of caution here.............................it's probably in your best interest to stay away from 150gr powder charges. They RARELY are very accurate at all. About the largest useful charge is 120gr of Pyrodex or Black Powder or 100gr of 777 powder. Any more than that just gets blown out the muzzle unburnt - and that affects the bullet's accuracy.

Here's a general rule of thumb that works with my guns:
350gr bullets use 90gr of 777 or 105gr of Pyro
300gr bullets use 80gr of 777 or 95gr of Pyro.

while all guns will be different - those are greast places to start.