I’m really conflicted here, with DU I’m worried mostly about it’s reputation as a “rich boys school” entitled etc. The cost is also something to consider. With CU I’m more worried about the general big college problems, huge classes, unresponsive administration, problems finding a good community etc. Financially speaking DU gave me $20,000 and CU gave me $3,500, this brings it down to DU being about 40k a year and CU being about $23,000 a year. I don’t really know what I want to major, which doesn’t help to narrow things. I’d be interested in some outside input.
My sister attends DU and she agrees that DU is full of rich kids and such. If you are planning to major in business, though, DU is a great choice. The business school is great and really sets you up for a job outside of college. I don’t know about any of the other programs. CU is nice too, but I feel it is much more of a party school than DU, and it is a stereotype of Boulder that it is full of potheads.
In the end it really depends on what you want. I would think that just due to its sheer size, CU would have a lot more diversity and places to fit into a community. It will also face the same problems of any big college. DU is smaller and as a result there are less people, but that also means classes are smaller and professors are more accessible.
Good Luck 
Have you visited both schools? If you have, it’s clear the campus & atmosphere are different. But I think when it comes to student body, they’re similar. Lots of well-to-do out of state kids at CU; lots of rich boys at DU. Lots of partiers at both. DU tends to excel in music, and has a well-connected business program, but it’s not well known outside of the Rocky Mountain area. CU has its party school reputation, but is also outstanding in the sciences, space-related research & design, and engineering. (Among the top ten for federal research dollars.) Both schools have good humanities programs. But because it’s larger, personally I think it’d be easier to find your “community” in Boulder - more people to pick from. Yes, it could be a tad harder to get classes as a freshman/sophomore, but most of the kids I know had no problem graduating in 4 years.
If you’re really unsure, go with CU. You’d be saving $17,000 per year – that’s $68K – lots of money for grad school, study abroad and skiing…
If you don’t know what you want to major in, don’t spend thousands and thousands of dollars more than you have to, unless you can afford it.