Doesn't matter if it's tags or categories or custom fields, what vikassah wants is to compare 2 (or more) posts.
So somewhere in some post listings there should be a "remember this post" checkbox, and then we'd need another "show remembered posts" link that will display the posts the user checked, allowing for a side-by-side comparison.
I'm actually workin on something like that, or actually, looking for a way to do this :)
We used it in our old CMS which was custom-built for the user to check certain speakers we had in our speakerpool and then have a list of "favorites" where he could then compare audio samples and see which speaker fit for his production.
Basically it is a sort of "shopping cart" system, without the checkout, obviously.
Any other insights, maybe?