I'm sure it's been said before in other threads, but a heavier duty drill is preferable for widening caves, where you are likely to want longer, larger diameter holes (subject to choice of widening technique). These heavier drills are more likely to have a hammer only function.
For bolt climbing or exploratory rigging, something much lighter is usually preferable, as it's easier to handle and you'll generally only be drilling short 8mm holes. The hammer only function isn't usually an option with these lighter drills, but that's not really what you'd want to use them for anyway.
Horses for courses.