From what I've seen of their books in the past they use too many pages just showing basic stuff like game covers, basic sprite art in huge size etc. Maybe I'm wrong here and this will be awesome though. I really do like those geek line books and wish they had them in English too, those are just insanely packed!
Bitmap books are more coffee table art books than compendiums. The strength of this is that the emphasis is on presentation - this is what the people behind it are all about, and it generally shows. They aren't writers. To a certain extent this is totally style over (prose) substance - but that's kind of the joy when you just want to communicate how/why something like this is so compulsive.
Geekline's offerings are a bit more like an offline hg101 or something. The strength of this is approach is that they are quite comprehensive, and offer a fuller (subjective) explanation of their subject. The downside is if you don't quite 'agree' with their interview choices, or editorials, it can kind of spoil things.
This metal slug book is hopefully the kind of second bite of the apple that bitmap earn when they strike up a relationship... we'll see what they can do with it.