With what it sounds like you are doing - it is best to CREATE your own landing page and send them their. I say this because you are using YOUR own money (PPC) and this will leave you with NO CONTROL. However, if their conversion rates are super high already - I would propose this:

Get creative with your own landing page and think a bit outside the box. I have seen people post "reviews" about the product they are selling to be very benefit rich and personal (sort of like a testimonial on steroids) then drop their affiliate link at the bottom so your prospect can check it out. This way, their offer is 2X as strong - and 2X as likely to make YOU money.

js

definately your own landing page, if nothing else to use for presell and better tracking of your results

If you're not working as a traffic broker (you're not paid per click), most definitely you send traffic to your own landing page. If you make money by selling stuff, make sales, don't broker the traffic.

Two things: First, like Iamkassi said, the pre-sell route means you can capture names and build a list--direct linking doesn't allow you to do that. Second, having a pre-sell page allows you to offer bonuses to those who buy through your links. The bonuses you offer can often make or break a sale. If you're getting PPC traffic, the prospects already have an interest in the product, they may have already made the decision to buy. But if you don't offer them a great bonus--and I do--who do you think they would buy from?

Forget the direct linking thing.