Types of Roasts: Choose roasts with a good amount of fat, both on the exterior and interior. Fat adds flavor and moisture to the meat.
Best Beef Roast Cuts: Chuck roasts are always good, and usually are less expensive than other roast cuts. Becasue this cut contains more fat, it should come out juicy and flavorful.
I am not a big fan of sirloin roasts because they oftentimes do not have enough fat on or in them to keep them moist during cooking. You can remedy this by wrapping them in bacon, or securing planks of beef fat to the outside of the roast.
Rib Roast--one of the most expensive cuts on the cow. It's what is commonly called Prime Rib. You can get it either bone-in or boneless.
Pork Roasts: Most generally, all pork roasts are good. Be especially careful about loin roasts however, because they are very lean. Look for pork roasts which have fat dispersed throughout the roast.
There are many different beef and pork roast cuts. Refer to Beef and Pork cut charts to help you in discovering what's available.
Seasonings: Beef will withstand bolder, more savory spices than pork. There's nothing wrong with good ole salt and pepper. Add some garlic or rosemary if you prefer. Coat the roast with a light coating all the way around to ensure uniform flavor.
Don't forget that you can also marinate roasts. If using an acidic marinade with beef, don't let the meat sit longer than six hours in the marinade, otherwise the meat will turn grey when cooked.
Salad dressings, beer, wine, are all good marinades.
Cooking: Always use a good quality meat thermometer to ensure that you don't over cook the meat. Cook pork to at least 140F and beef to at least 130F, By letting the meat rest before carving, the internal temperature will raise by 5-15 degrees.
You are roasting. Keep the grill temperature between 250-300. Figure that the meat will cook at 40 minutes per pound at 250F. If you want to add smoke, do it in the beginning of the cooking process, when the smoke will adhere to the water molicules in the meat.