I own both books and would say that Penton's book has a more negative touch. Of ocurse, as an XJW Penton remembers that the JW claim to be God's only religion and this is the benchmark he uses. Garbe sees them as just another group that was persecuted but was outstanding in some way. To me, Garbe collected more details and has more internal distance to the topic. Penton does a lot of ad hominem against JW apologets (which may be deserved). When reading Penton I did not find anything new because I had read Garbe before.
Some claim Garbe is pro-JW because he spoke at the "Stand firm"-exhibitions and because he changed some things in later edition after having access to the Watchtower Archives. But you have to notice that he also rejects the idea of JW being an "example for today" as they have claimed in Germany.
Garbe's data are found here:
http://www.clio-online.de/forscherinnen=4156