HU is the best review site imo since they cover a lot of games in their test suite and generally don't waste time ranting about useless stuff. Not sure how GN is more popular when they barely test 3-4 games in their gpu reviews.
For speed, Linus is the best imo - he gets through the review quickly with just enough detail to get the gist of it. Both GN and HU reviews just drag on forever and ever.
As to why GN is more popular, it's simply - they offer a ******** more content. HU is very narrowminded in it's focus and they clearly have a resource issues for some time now in creating new content. GN puts out a lot of cool videos like factory tours, hardcore overclocking, how PC parts are made - the more content you offer the more subscribers you will get. Linus didn't get 10 million subscribers by just reviewing graphics cards, you have to expand and until HU does that their subscriber base will be much smaller