I will start by stating that I prefer the unabridged edition in comparison to the abridged edition.
However, whether or not you ought to read the unabridged or abridged version would fundamentally vary on the experience you're oriented towards in the tale. I do not wish to spoil any potential story elements, although it can be noted that the unabridged version of The Stand is the text in which Stephen King finalized for the general populous in mind; it was upon Doubleday's (the company publishing the book at the time) request that he remove material as to lessen the overall cost, since the 1,000+ pages would consequently up production fee and compare relatively poorly to his other works strictly in terms of finance.
Thus, to clarify any potential misconceptions, the unabridged version was not published merely as a text whereupon the additional content was only mediocre; King killed his darlings with the abridged version (which, currently, is not even being manufactured anymore, as to my knowledge) and decided to revive them due to profound amounts of request that he expand upon the already staggering tale. As such, there are ultimately extra scenes and exposition, which I loved because the already well-developed novel simply seemed to benefit even further from the new brandished content. To speak in greedy terms, it allowed me to be encapsulated within the world for far longer, and to me that is but a positive.