Is there a big difference in taste or quality when using LME, compared to DME?
Not really, if the LME is fresh... but in the cans it can be hard to tell how long it's been sitting on the shelf. If it's old it can taste like ballpoint pen ink. DME, as long as it's powdery, you know it's been kept dry and has a longer shelf life.
The guy behind the counter said I should get Muntons dark LME since I was brewing a stout, recommending it over the DME. Hope he wasn't trying to push old stock on me.
He probably
wasn't trying to push old stock on you, he was probably trying to sell you the easiest way to get a good result. The reason we usually recommend using
pale DME and coloring and flavoring with specialty grains, rather than using
dark extract is that this is more similar to the way a professional brewer brews. The great majority of the grist in
any beer, whether it's a black stout or pale pilsner, is pale malt. Using pale extract + specialty grains mimics this and gives you control over what's in your beer.
Dark extracts will usually make a fine dark beer, but the manufacturers almost never publish the breakdown of what malts were used in making them. Also, it's hard to use leftovers for anything other than dark beer, whereas light extract can be used for anything.