I'm playing around in the same area as yourself. I believe a sump is great for mechanical filtration as well as a great place for equipment, protein skimming, heaters etc. I'm not a seasoned pro and I'm sure if I'm wrong I'll be corrected; a sump setup with our thinking is referred to as a plenum. Lots of reading here on it
http://www.wetwebmedia.com/PlenumFAQs.htm
A refugium is a seperate animal all together. Many will run both. The refugium is suppose to be on display as well. Setup beside or behind your main tank etc. The plenum idea and DSB issues have been beat to death on all the forums. The refugium setup seems to have its place and escapes much conversation other than being praised. My nitrate levels and water parameters remain quite constant. Nitrate is always below 5 etc.... I may concentrate on other areas myself, maintaining calcium and alk is climbing to the top of my list. How are your parameters?