Hello,
I'm not sure whether you wanted an explanation of the differences between advertising, marketing and PR but I thought it might help!
An ad agency will create, design and often place adverts across different media.
Marketing agencies (and some ad agencies) will advise you where exactly the ads should be placed (ie which TV channels, websites, newspapers, magazines, billboards etc) and also place them. Some will also do the creative execution too.
With both of the above you pay to have your exact message placed where you want it in a title of your choice and they will do this for you. They will come up with a whole campaign with the same message running across all media timed to break at the same time.
PR agencies will come up with a news-worthy story (or drip-feed campaign) based on your business and what you are doing at the moment (or through a pre-agreed length of time). They will write and send out press releases and ring-round news desks and journalists they are in contact with. The newspapers/ magazines/ websites may run this story and they may not, they will also have free reign to write what they like about it so you might not get the message you want to say across and it might not be positive.
It is often argued that PR is more effective as it comes across as information and people actually read it and take it in, rather than an advertisement which they can easily ignore. However it can badly backfire if the journalist does not agree with you or sees some negatives in what you are saying. You can also get misquoted and misinterpreted.
My background is journalism and PR and my DP owns an ad agency so we often argue about what is better! Personally, I believe a combination of both is the most effective way to ensure your campaign gets across. It's probably best to spend your money on a good advert and place it in the highest circulation magazines you can afford (see: [www.abc.org.uk]) and try and do some PR yourself. I'm happy to offer you some general PR advice if you like.
I hope this might help and I haven't told you what you already know!
Lulu x