How far do you sit (or PLAN to sit) from the TV? What is your normal viewing environment like? - Bright? Sunlight? Dark? Light Controlled (curtains, blinds...)?
If you sit beyond 8 feet, the Samsung should be ok: it is a "720p" model, as opposed to the 1080p screen on the Vizio, which might be a drawback if you sit close to the TV.
Everything else being equal, would prefer to purchase from CostCo: 90-day return policy, 2nd year warranty included in purchase price.
However, think BB has improved their return policy to 30-days, and recall seeing something about a "TV Best Fit" guarantee (or something like that) in their ads.
OTOH - and FWIW - CR rates the Samsung above the Vizio. (Their website - consumerreports.org - provides MUCH greater detail on the reasons behind each set's rating then the magazine does, although still not what you would expect to find from CNet, or other dedication home theater / electronics sites/magazines.)
You might have trouble finding reviews of these panels from other sources, as most reviewers don't bother to do very many "entry level" panels - but it would certainly be a good idea to try a web Search...
perhaps only $600 MAX with taxes included
Then again: if you can run up to $600, you might see what a 50" set would go for: perhaps a Panasonic U50 or even a UT50, or the Samsung E550, or even the E490 (although the 490 also slips to 720p).
Go back to "What is your viewing distance?"