Are you asking how to use Myriad Pro on your website or looking for advice on which would be the best weight to use?
If the former, you will need to either own a license for web-use of the typeface or use a webfont service. Adobe Typekit includes Myriad Pro. If you have a Creative Cloud package you may have access to Typekit and therefor can use it from there. They have clear instructions on the different ways of using webfonts.
If you're simply asking which weight is best to use on your site, then I'd have to say that depends on the design of the site, coupled with the amount of text (you mention there are long articles) and also personal preference. With the numerous different variants in screen resolutions you need to choose a typeface which is legible across all screens – last thing you want is a user not being able to read your article. Sometimes if a typeface is too light it can make it difficult to read.
Going back to Typekit, you can try out the different weights live on their site to get an idea of which variant you prefer. You also need to be careful with how many weights you decide to use. If you add bold, regular and light, for example, you're adding more bytes to your pages which will have an impact on load times. I tend to stick to two or three webfonts per site which may include a headline typeface and body, both regular & bold, or similar such as semi-bold and light.
Hope this helps a little.