The type of memory used on a particular system and for what application will have the determining factor for the most part. Mpst gamers and certainly OCers will go with performance not value ram while someone with an average or budget build especially prebuilt will see value ram there for things like occasional gaming, checking the mail, word processing and desktop appls, and of course some multimedia maybe added to that.
For choosing a brand of memory you then have to compare the quality there. Names like OCZ, Corsair, Kingston, Crucial, Mushkin, and a more recent name Patriot are the more commonly top shelf brands there. For the HP there you would go with the most compatible and reliable value ram since you probably won't be pushing the case there. Corsair made the list above.