I have only ever used to Mosby and flipped through the Lippincott guides so that's all I can comment on.
Short answer is that between those two I definitely prefer the Mosby pocket guide. It seemed to be easier to use and (I have nothing to prove this) more thorough. It's what I used all through nursing school and I loved it. Of course, with any pocket guide there are going to be drugs missing from it. That will be my one comment, when using the Mosby guide in school I found that somewhat frequently drugs I wanted to look up were not in there. Now, if this is a situation unique to Mosby or not I don't know so it may not be a factor to deciding.
I am assuming you will have your drug bible (Whatever the American equivalent of the CPS is) there as the final word?