Can't Decide On 46" LCD TV: Sony or Samsung

[Kage]

I have a 3 year old, top of the line 37" Sharp Aquos 768P lcd tv, which is located in my bedroom and I will be buying a new tv within the next 2 weeks to replace it and the new tv will be a good birthday gift for me. Since the beginning of this month, I've been looking at 40" LCD tvs from Samsung (650) and Sony (Z4100). Now I feel that I'll be happier to get a 46" LCD instead of a 40" LCD, because it will be a noticeable difference in pq from 37 inches to 46 inches.

Here are the objectives that I want:
1. Must produce excellent PQ in blu-ray movies and in PS3 games.
2. Very good build quality.
3. Reliable brand.
4. A solid 1080p24 playback.

I'm still considering to purchase either the Samsung LN46A650 or the Sony KDL-46Z4100, but its a difficult decision to make. Which online store is a good place to buy a tv from and that has an excellent return policy (i.e free return shipping). My local Frys has these two tvs on sale. Should I buy the tv locally? Should I buy an extended warranty for the new tv?