DTS is generally regarded as superior, but "better" is often confused with "louder", DTS output is often a little louder than DD which often makes people assume the DTS mix is better. The extra volume from DTS is especially apparent from the rear channels, I always bring the rears down a notch when watching a DTS movie as they can often be a little overpowering.
A great DTS mix will in theory outperform a great DD mix, but there are many examples of films which have had a superb DD mix which is superior to the DTS mix (to those that can actually notice a significant difference!) DTS was designed for 1.5mbps but is often compressed to half of that amount, where as DD is variable, and therefore more often than not can handle anything that is thrown at it.
If space is ever an issue go with DD, chances are you will never tell the difference.