It is still waaaaaaaaaaay better than asking your family and friends.
They're both 100% useless.
Mechanical Turkers are incentivized to give positive feedback because there's a chance (lurking in the back of their minds, surely) that people are emotionally sensitive and if they get criticized, they will be emotionally wounded and won't approve their HIT.
Do you know how I know this? I asked Turkers to give me feedback on one of my sites, and it was nothing but sunshine and razzmatazz smoothies.
You know you're building something people want when they vote with their wallets or attention (give you their email address, phone number...)
New Theory: hell, an even better test might be are they willing to "invite their friends" to tell them about it? People share things with friends because they like their friends - is it of sufficiently high value that they will get a "social credit" for sharing with their friends?.
You could compare a few pictures, videos, landing pages, etc. And get some a vote with a comment on which is better.
Forced more legwork on the user to actually create multiple options.