by going with either of these (/xxx/pr-123.php vs /xxx/www-pr-123.php) you are signaling to the search engines that "xxx" is the common factor and each page within that "virtual" folder would then be seen as a sub-set. So the question then is whether "www-pr-123.php" or "pr-123.php" is more appropriate for SEO purposes.
I'd say that it all depends on the importance of the "www" portion. If "www" is in fact a relevant word/phrase, either from a general search value or from a brand identity value, then it's best practices to include them in the URL. If they're neither valued for search by people who are not familiar with your specific products, or alternately, not important to people looking up your products who already know them, you can leave them out.
Another take on this would be - what's the chances someone would search for the products by part number without the www portion? And what's the chances of someone searching for the products by the "www" portion only?
While it's not necessarily critical to include every single aspect of a part's known naming convention in the URL (if you have optimized page Titles, h1, content, image names, etc), I'm guessing you want to cover the bases in how people search based on different situations, so in that case, don't leave them out.
As far as the concept of having too many URLs saturated with any aspect of category, product name or whatever, there's no need to be concerned with that if they're valid products grouped properly. It's more important to provide unique quality content on every single page. And if you're talking about many very similar products, that itself should be your biggest worry.
Just having the same descriptions repeated over and over page to page, with just a single word or part number being the difference is what would be the problem. Coming up with 100 or 150 or 200 words that are uniquely written for every product - now that's the bigger goal and far outweighs the URL repetition.
Then there's the fact that if you have all those repeated URLs, you're actually telling search engines "we've got a lot of relevant products for that topical focus". But only if you get those unique descriptions done right.