My favorite of the PPC engines is certainly Adwords. As someone who generally spends less than $1000 per month on PPC advertising, I am not too concerned with having an upgraded account, or anything similar. Generally, I just go after the keywords that I am looking to convert, which doesn’t require the input of any of the engine’s support staff.
Much of what I do is assist in the development and launch of new web stores. The sites I create, and/or help others develop, are usually targeted at specific niches. None of my clients are looking to create the next Amazon.com, they are usually just going after small communities of which they are a part. Normally the targeting I do covers specific items. If someone is looking for model XYZ123 of something, they are most likely looking to buy, usually based on price. For this reason, I have had good luck with going after potential customers of very specific items. As a result, Google takes the cake. For the majority of these items, the PPC traffic from paid search results rocks. On the contextual side of things, when people are looking at a review of a specific item that is highly rated, they are generally looking to buy.
With Google’s PPC sitematching, and the variety of tools you can use to pick and choose your ad spots, it makes it a win-win situation, specifically for retailers who sell items that can’t be found on conglomerates such as Amazon, Overstock, etc. Just go after items that have glaring reviews when doing context ads, and go after items that you can do well price-wise for search. There isn’t much more to it, in my opinion, specifically on the retail end.
As simply a traffic source, I’m not sure which I would pick. Most likely Adwords due to the sheer size of the potential customer base. The more people there are for you to reach, the better your chances of making a substantial dent in your traffic goals.