Not Apple bashing (I'd do the same regardless of manufacturer) but in all seriousness if it's under a year old take the receipt and demand a brand new one. You're within your rights to as the goods sold to you were not fit for purpose - check the Sale of Goods Act.
Just a heads up - but this is completely wrong! You have no legal right to a brand new machine. You have the right to repair, replacement or partial refund, up to 6 years after the date of purchase. Before 6 months, it's down to the manufacturer to prove there wasn't an inherent fault - but after 6 months, it's down to you to prove their was a fault at the time of purchase.
Either way, you're never *entitled* to a straight replacement. And you'll get further being polite than going in demanding stuff.
As for the Samsung/LG thing - it's a mixed bag. Not all LGs suffer with image retention - and if you get a good LG, it's by far a better panel than the Samsung. Some people seem to hate on LGs - there's guys over on Macrumors who have had 15 replacements simply because they're LG displays - whether they suffered from image retention or not. Feels like you're banging your head against a brick wall trying to reason with the guys over there sometimes!!!