Both actively pursue potential authors. I know this because I’ve been contacted by both. Made me realize I have neither the will nor the patience to write a book - being an author sounds great on paper (pun intended).
Manning seems fine to me. I've read a few books from them, from quite good to blah.
I've read one book from Packt that was quite good; they've also published literal plagiarism (which they withdrew after the original author, a friend, tweeted about it -- this wouldn't have come to my attention otherwise). Can't recall seeing another that interested me, flipping through the pages.
It's a shame because there is good stuff in their catalogue, but it's littered with trash.