This is a great question, as there are different options for creating niche affiliate sites.
A mini-site has various definitions, from the small 5-10 page website to the landing page or squeeze page style affiliate site.
A blog is of course content-based. You can also use WordPress or other blogging platforms to create squeeze pages or more traditional looking websites. But the platform is irrelevant. A blog is a blog. A landing page created with WordPress is still a landing page.
An ecommerce-style affiliate site looks very much like an online store, except that you deep link to the merchant's product detail/checkout page instead of linking to your own shopping cart...
Which type of affiliate site should you create?
The type or style of affiliate site you create is going to depend on several factors. Your market, your choice of keyword phrases, the type of product you are promoting, and how you reach your market - meaning your marketing strategy.
Ecommerce-style Affiliate Site
This style of site is used for promoting a line of products within a specific niche, usually from multiple merchants. An example would be an affiliate site on exercise equipment, or kitchen appliances.
Pages are optimized for specific product types and specific product names. These keyword phrases have commercial intent, and the searcher is looking for purchasing options, reviews, comparisons, the best merchant to buy from online, etc.
In addition to product listings with images and descriptions, this style of affiliate site would also include content pages to help the visitor make a more informed buying decision.
A blog is basically the same as the traditional content-based site, with the added features of visitor interaction. The goal is to establish yourself as an expert on that topic, and build a loyal readership in your niche.
Posts are optimized for informational keyword phrases, and the content is written to answer those queries and follow-up with a specific call-to-action. This method takes the target market from searching for info to choosing a solution.
If your niche has a good combination of informational keywords, and keywords with commercial intent (product names, etc), you might consider adding a blog to your ecommerce-style site to capitalize on both.
A mini-site is a smaller website, arguably anywhere between 3 and 20 pages total. This type of site can be created using the WordPress platform, or traditional HTML.
The affiliate usually has no interest in creating a name for themselves in the niche, but rather to send the visitor from the page to the merchant as quickly as possible (ie high bounce rate, very little personal interaction).
This style of site is generally used in small niches that do not have much market depth. For example, a website on the topic of wheatgrass juicers. Your content will consist of product comparisons and reviews, choosing between manual and electric, and perhaps a few of the best recipes to be used with that product.
Affiliate Squeeze Page
The squeeze page method is used to build a niche mailing list for the purpose of email marketing. This style affiliate site consists of only a few pages, with the main page offering an incentive that the visitor must request via email.
It is most often used to capitalize on informational searches, and then gradually bring them to a buying decision.
You might use an audio interview with the merchant, a case study on the product, a how-to guide on a topic that requires product purchase, etc. You can then follow up with the subscribers and build a relationship via email, recommending specific products related to that topic.
You can use an autoresponder series to automate the process, making this style of promotion more passive than blogging and interacting with blog readers.
Many of these models can be used together in various combinations. The most important factor in your decision about which style to use is what will work best in your particular niche - or with the keyword phrases you target.
In an informational market for example, you might use a blog to become known in your niche and then also offer a squeeze page and free download to funnel those visitors into your mailing list.
You want to research your market, and consider the available keyword phrases in that niche, and consider what you can create to best serve that particular market.
Next we'll discuss the use of pen names with niche affiliate sites...
p.s. I'll be presenting at the Niche Affiliate Marketing workshop in Atlanta this January. There are still a few seats left if you're interested in joining us! If you can't make it to a live event, but would still love to learn niche affiliate marketing, check out my overview of Rosalind Gardner's Super Affiliate Handbook.