I also found that the money spinners made the game a bit easy. In GT3 I had to work harder for the cars I wanted, and I thought that was better. On the other hand, I got to about 45% on GT3 and stopped there, whereas I finished GT4 (b-spec also helped), golded all licenses, and still have tons of fun on it, so on that evidence, GT4 is the better format.
I'd think the game would be more challenging if the prize money was higher and they got rid of the prize cars altogether, so if there was a car you really, really wanted, and it was expensive, you'd have to work for it. Doing the same races over and over again doesn't have to be repetitive if its challenging. I used to race the same 16 circuits in Grand Prix 2 over and over again, against the same opponents, and never got bored, but a 2 lap race is a bit different.
Yeah, if you've used the cash cow events, and suddenly find the challenge is gone, I can understand that. It's been said that you don't HAVE to abuse it, but that doesn't really help because you don't know if its going to make you bored of the game until you've done it.