I'm interested about this too! DU is my top choice, but I currently live in Chicago so it'll all be a bit new to me. I visited DU and did a little research on both schools on my own so here's the little I gathered. That said I would love to hear about DU from someone with more first hand knowledge and experience there!
It does seem like DU has more options in terms of certificates and specialization. DU's school of social work only offers graduate and Phd programs, no undergrad degrees, which I thought was interesting. Not sure if this has any effect on research/professors/ect, maybe someone else will have more insight there. Generally speaking, bigger cities also have a wider variety of field placement options so if you know you want to try out a specific area of social work Denver could be the right choice.
The thing that made the biggest difference to me (and the reason I applied to DU and not CSU) is that CSU has a requirement for the amount of direct social work experience you must have. From my meeting with an admission's counselor at DU, it seems that they are more open to students from different backgrounds (although social work experience is always a plus obviously!) as long as your interest in social work is serious and genuine. Since I'm coming from a communications/marketing background, this was important to me, not sure how much it affects you.
Good luck on your apps! Maybe I'll see ya at DU