I hired a car with driver, and a guide, the first day, because I'd heard from others on various sites that a guide was important in getting the most out of the temples. I started with a half-day afternoon (which was way too hot) and lasted about 4 hours in the sun.

For me, a guide wasn't good. I got quickly overwhelmed with details that I didn't honestly care about at that point, and felt like I had to absorb way too much when I really just wanted to look and explore and move quickly through things. I didn't actually care about what king built which temple, turns out, or what year such and such was constructed. That's just me, I'd been through a lot of history and it just seemed like some incredibly hot and humid and boring history lecture, not what I was wanting.

The second day I tried a tuktuk and a driver alone, and that was great! Again, great for me, maybe not you, but the moral of the story is don't believe any who says you have to do this, or have to do that, to get the most out of things. I preferred the tuktuk to the car because I felt closer to things, and really, I spent 90% of my time (or more) on the ground, walking around, and 10% of my time in the car. And when you first turn a car on it's hot anyway, the a/c takes a while to kick in. An open-air tuktuk cools down (such that it cools, not as much as a car obviously) immediately with the breeze as you travel.

Just my two cents.