Plasmas have a "softer" picture, so generally Plasma's are MUCH better for SD picture.
If I could have afforded it, I would have purchased a Pioneer plasma - they are fantastic. Unfortunately their price reflects this.
I've only had my Sony 32" 100Hz CRT TV for around 4years, so the fact I've bought another tv now means I'll probably upgrade again in 4 years. In the meantime, this TV will do wonders for my Xbox, DVD's etc.
Obviously this is all down to personal preference - as said before many people will not be happy with LCD for SD, but I suppose it depends on what you're used to or how picky you are with the picture.