If you entertain many people at once, you'll need a bigger grill. If not, a smaller grill may be a better choice. It will cost less and take up less of your precious deck and patio space.
Look for quality stainless steel and porcelain-coated metal grates. Heavier grates sear meat better so it holds in more flavor.
Grade 304 stainless steel is the standard for grills. Check with a magnet. It shouldn't stick. Porcelain-enamel finishes are also rugged and come in attractive colors.
Door racks, hooks, and towel bars make a grill much more functional. Make sure the propane tank of a gas grill is easy to remove and replace.
BTUs (British thermal units) are the standard measurement used to state the amount of energy that a fuel has as well as the amount of output of heat. 100 BTUs per square inch of cooking area is about right.