Your friend is biased if he's putting you off plasmas.
LCD's arguably cannot rival the best plasmas for picture quality although some are now running them close - see the What HiFi S&V group test from Nov 2007 "The World's Best Flatscreens". Against arguably some of the biggest name LCD's on the market, the Pioneer Kuro 428XD plasma came top and the Panasonic PX / PZ 70's plasmas ran it close.
Some LCD's still have a tendancy to overblown skin tones / cartoony colours, motion blur and poor black levels although as I said, some of the very best are now quite close to plasmas in these regards so it ins't necessarily a problem - it just depends on which LCD your buying, however, in many reviewer's opinion, plasma is still king. The Sony 40W3000 LCD was excellent in that test and well worth considering based on that if going the LCD route. I'm presuming theres a 46" version with similar characteristics.
Most people still prefer plasma in larger sizes though.
Other worries from LCD are stuck / dead pixels and back light failure - a little heard of but potentially expensive problem usually resulting from either tube or transformer failure. Most LCD screens have a fluorscent tube behind them. I've seen a few people with this problem on the High Def forums and it can be very expensive to put right. Plasmas achiles heal is screen burn although with most tv sources and a properly calibrated screen this isn't likely to be a problem these days as plasmas have improved massively. Gamers might want to consider LCD though as high contrast / brightness long time static images such as game menus don't always sit well with plasma.
As for SD sources, it all depends on the individual tv's hardware. As a rule the greater the resolution of your tv, the worse SD sources display because the larger the picture, the more interpolation is needed (software guessing the missing pixels). Its like blowing up a still photo at home. At 100% its sharp if a good picture. At 200% quality reduces at 400% it can be quite markedly blurry. Ultimately though the quality does depend on the tv's hardware / software in making these guesses.
Also, screen size and viewing distance have an effect. A 50" tv can look blurry from 10ft with an SD feed yet the same tv can look great from 15ft! Change to a bluray source and you might be able to view it from 6ft! As a general rule, the better the source, the closer you can sit without noticing imperfections.
The Pioneer Kuro's (428XD) is very good with SD.The Sony 40W3000 got a good review with Freeview pictures in the November test.
As you ask about Samsung, the LE40M86/LE40M87 didn't get such a good report in the Nov test for its tv tuner. So maybe it needs careful consideration if that was your proposed model.
I suggest the best answer might be to buy the Nov 2007 edition and check out the full test results for yourself and take things from there. You also need to factor in your viewing distance.