Someone told me yesterday about a study, which they heard on TV news, that claims to show that non-monetary incentives are generally more effective than cash incentives for motivating employees. The theory is that employees are already making money, and most people are more apt to get excited about a different kind of reward (tickets to a sports game, for example) than an extension of the same reward (more money).
If that's true, then we could expect the same psychological pattern in games. Generally, you'll get more bang out of a different kind of reward than more of the same.
That seems right to me. Variety of rewards is always good. The hard part's ensuring every reward can be appreciated by all.