Know-How Exchange : Branding

Value propositions last and confer (surprise, surprise) VALUE.

Tag lines change over time and, for the most part, say nothing of any worth, merit, or value.

Most tag lines are a waste of space: for the most part all they is bamboozle.

MMM MMM GOOD – Campbell’s Soup (so what)?
IT’S THE REAL THING – Coca Cola, 1970 (compared to what)?
TASTES GREAT, LESS FILLING – Miller Lite (compare to what)?

The only product I can think of at the moment that has a tag line that adds any value is Geico, with its refrain of "Fifteen minutes could save you 15 percent on car insurance".

@Gary Bloomer: I think you are right, I just want to clear up the idea.
You are talking about two companies who were competing with powerful rivals for same market: Coca Cola disputing the largest marketshare with Pepsi(who were quite tenacious as it turns out) and Royal Crown Cola (of course there aren't many teenagers who know this brand), while Miller Lite was trying to enter the light beers american market dominated by Bud Light (consider the blind tasting studies put Miller in great dissadvantage).
On the other hand, Campbell's Soup uses their "Mmm mmm Good" SLOGAN since 1935 till present day. What it communicates is the promise of a tasty soup, the feeling of it, therefore it's not only a slogan and/or a tagline, it's a value proposition too. And when you take the thinking to a deeper level, it's actually a brand.

Normally we see value propositions following the rule "better, cheaper, faster", but this type of marketing was productive in the 20th century, nowadays you can't set yourself apart from your competition using this technique.
Secondly, many see taglines as something smart, something witty that sounds good along with your brand name, as Gary mentioned, they seem like bamboozle. Unless...
Unless you ad value to them. If you start your thought process from scratch, you'll reckon the question shouldn't be "which is better: taglines or value propositions?". Instead you should start with "how I bring value to my clients?" The answer to the former question will lead you immediately to a tagline or value proposition.

Ex: Connecting people - Nokia (just brilliant), Think different - Apple, Live richly - CitiBank, LG - Life's good, Impossible is nothing - Adidas, Because you're worth it - L'Oreal.

The comments above are right on.

Another distinction is that the Value Proposition is used *internally* as a guideline for Positioning and communication. It isn't usually written with consideration of direct customer or consumer communication. A Tagline, in contrast, is specifically written to reach customers/consumers in terms that will be most meaningful to them. When done right the Tagline expresses the essence of the Value Proposition -- in consumer language.