curves allows more precise adjustments to the pic. levels is simpler but not as precise. up to u to play around n see what u prefer to use. i use curves when i want to be more precise and levels when i cannot be bothered. lol.
Levels give you an easier view of histogram. If you just want to adjust white point and black point, levels would be easier. This is usually sufficient for most photos, alone with mid point adjustment. Curves give you more finer control over the whole range by letting you define individually the way sub regions of the tonal curve behave.
Level does not easily cause clipping, neither curve does that. You are in control how much contrast and brightness to be added.
Books by Dan Margulis have alot of examples in using curve for color and contrast correction. I recommend reading :
1) Professional Photoshop Fifth Edition: The Classic Guide to Color Correction
2) Photoshop LAB Color: The Canyon Conundrum and Other Adventures in the Most Powerful Colorspace