I think optimal bet sizing (“amount,” in @zmansuncle’s original post) is more important, because it requires a solid understanding of when to bet/fold/call. Let me provide an example.
Let’s imagine I’m at a loose, passive table, like most of those featured on this site. If there are going to be a ton of preflop callers, basically regardless of my bet sizing, then I’ll want to fold almost all of my hands, and bet extremely heavy when I do have a solid hand. If I’m calling too often, then on most boards I’m going to have no idea where I stand; if I do catch a healthy chunk of the flop, it may be difficult for me to get much further value. On the other hand, if I’m betting large too often, then I run the risk of getting called or raised by much better hands. Keeping my range tight influences my decision to bet big, and betting big means I have to keep my range tight.
Another example: With 100BB effective stacks at a 9-handed table, I followed the advice from above, betting 7x on the button facing 4 limpers. The big blind, UTG, and CO all call my large raise, resulting in a pot containing about 30BB after rake. The flop comes Kh 9s 2d, and it checks to me. What should I do? With such a disconnected board, I like c-betting with my entire range. However, if I bet the pot with my entire range, then my opponents can profitably call or check-raise whenever they have a king.
The bet size is too large to get calls from draws, since the only draws on this board are backdoor flushes which shouldn’t chase a pot-sized bet, or an inside straight draw (JT, QT, or QJ). Also, because of my raise size and relatively tight opening range, I’ll have more combinations of low-to-medium pocket pairs (possibly up to 30 combos if I open as loose as 44), TT-QQ (18), and suited aces without a king (anywhere from 4 to 16, depending on how tight you want to be) than 99 (3), KK (3), AA (6), or AK (12), leaving me in rough shape when I get called by a competitor with a king.
Betting small - in the realm of 20-30% pot, or 4-6BB - will garner folds from low suited connectors, aces that missed the flop, most low-to-mid pocket pairs, &c. If you check here, you might allow someone with weaker draws to start catching up (e.g. 67s having an OESD if an 8 comes on the turn). This could allow you to scoop the whole pot for cheap on the flop, picking up over 20BB with your entire range. Meanwhile, when you do have AK or 99, you’ll be able to get value from weaker kings.
Now, what if you only minbet here? Well, nobody should be folding, since just about everyone will have at least 3% equity against your range, even if the flop apparently missed them. All you’ve done is further bloated the pot.
Long story short, bet sizing requires an intimate understanding of the decision of whether to bet in the first place, and is therefore more important.