Is it better to have a landing page for the affiliate offer(which has a link to the offer) or is it better to just directly link to the affiliates?
What do you mean by "landing page“??Probably you mean that you create a page like "http://www.youdomain.com/123.html" ,then redirect it to the affiliate link? No need to do that,just use htaccess to redirect it..(i.e,you could use your own domain ,and it'll redirect to the affiliate link directly,no planding page needed)
Direct link is better, if advertiser allows it, landing page filter the traffic, less than 50% traffic reaches merchant site from landing page.
if you do ppc, and the advertiser's page isn't very descriptive, ie a large graphic, then google will penalize you... for google ppc I find that creating my own landing page and stuffing it with the right keywords avoids me having to bid $10 for keywords.
In many cases, if the visitor is not 100% sure what he wants to buy, then landing them on landing page or review site about the subject could increase conversion from 50 to 500%. As with everything, it depends from context. If your visitro comes from keyword: "buy blue widget cheap oklahoma", then get him straight to the right page. But if visitor is looking "best widgets", then there is room for a solid review page or site, explaining and leading visitor to *best* choice. my 2 cents