Both churches are so pristine and I would say timeless - look at the perfect assembly of columns in the transept of St Michael (PHOTO)!, but this is sometimes misinterpreted that they are sterile. Another frequent issue concernig this site is its authenticity, I would comment it that I do not see it as an obstacle, because the most of medieval churches and ancient temples as we know them now are just current interpretation of the original state.
I visited both churches two-times in 2008 and 2009, thus it was before extensive restoration of Mariendom (cathedral) in 2014. So, I would like to visit the site again as soon as possible.
St Michael: I like the dispostion, the assembly of columns, the combination of columns and pillars in the main nave, the use of white and red in the entire interior, and extraordinary wooden ceiling. The church is a culmination what has been started by a bit older St Cyriakus church in Gernrode (close to Quedlinburg).
Cathedral: the disposition and shape of this church is not such perfect as St Michael, but it contains world-class masterpieces such as Bernward doors and column, Hezilo and Azelin chandeliers, and baptismal font. And do not miss the thousand-year rose...
Other worth-visiting places in Hildesheim: reconstructed old market square with Knochenhaueramtshaus, and romanesque St Godehard church, which is more "authentic" than WHS-churches (it means with 19th-century colorful decoration) and it is located in the part of town not destructed in WWII. I would not be against to include St Godehard to this WHS.