Are there any difference if we submit articles to an article directory PR-2 and PR-5? Do they generate same back-link quality?
When the "Google Spider" looks at your backlinks it looks at the number of links pointed to your site but also, and more importantly, it looks at the quality of those links. One high quality link from a strong relevant source can be worth as much as dozens of lower quality links. The number and quality of inbound links that a given page has is a large part of the Google ranking algorithm. A site that has a higher page rank than its competitors will likely show up higher in the search results. Contrary to popular belief PR IS NOT a complete measure of how well optimized your website is. Search Engine Optimization consists of more than just link building. This is why you will often find some sites with low page rank that show up higher than pages with higher Google juice. - The idea is to optimize your page and then to get a solid rank.
Well it might depend on the popularity and the traffic, alongwith the submissions done to the directory per day. The more highly ranked and better rankings in SERPs directory will get you more valuable backlink. BTW pagerank really doesnot matter, because it can be easily manipulated by just buying high pagerank links, so better you check out the popularity of those directories
This is an idea that shows up in tons of posts... but I don't really think you can compare search results based on pagerank. You have no way of knowing what the current pagerank is (unless you've hacked into Google). The value showing in the toolbar is probably a year old or even older. The pages in question may never even have been updated at all or may not have existed at the last update.