I can't say what the best long term strategy is, but I can say that as a tech journalist I almost always prefer to hear from someone involved in a company rather than a PR firm. But hiring a full-time internal PR person isn't the same thing as hiring an agency.

As inthewoods wrote, the reason for hiring a PR person is for their contacts. Agencies will make a lot of claims about how many contacts they have, but I've heard they'll often stick low-priority accounts with junior PR people or even interns (and the quality of pitches and press releases I've seen from well known agencies about unknown companies seems to confirm this).

Jagermo wrote: "i get a lot of mails from marketing people who think they know what journalists want. Most of them do not."

I get a lot of e-mails from PR people who think they know what I want, and few do. I get a lot of the same from marketing, but a lot of journalists end up in PR or marketing at some point in their careers and they often do know what other journalists want. Some people just seem to get it, whether they've been journalists or not, and some don't. I imagine it's easier to keep track on who's doing what and how good they're doing if they're internal and not outsourced to an agency, but I wouldn't know.

FWIW, some of the best pitches for startups I've gotten have come from investors.