Assembly is easier than ever but it does require space and some time. If you're mechanically challenged don't worry - many retailers now offer their grills fully assembled for a small charge. Now that Blue Rhino cylinder exchange is available at most grill retailers, you can purchase your grill assembled with gas and start cooking immediately. Bigger is not necessarily better. Almost all gas grills have enough power to sear meats and enough control to cook delicate foods.
Quality is the difference.
Structural strength and durability are key. You'll use your grill frequently. Most frequent grillers prefer cooking hoods without windows. Glass can break and requires frequent cleaning. To control your cooking temperature get a grill with an accurate temperature gauge.