RG6 vs RG59 HD-SDI cables for live stream It seems there are lots of discussions on the web, none recently though, on RG6 vs RG59 and the distances that they can carry signal.

For situations when quality matters, such as live streaming (as opposed to something like CCTV), is RG6 the only way to go?

A 984' RG59 cable costs just about as much as a 200' RG6 cable. Assume that the signal is powerful enough to send the signal over 900'. Will the RG59 cable experience significant and noticeable signal loss compared to the RG6 cable? 

There is a significant price different between the two types, so if I can get away with a RG59 cable, certainly I will.
Unfortunately there is no perfect or exact answer. Your question topic mentions HD-SDI, which is a digital signal. Those tends to degrade 'cliff-wise', unlike analog signals where degradation is gradual. It will partly depend on the quality of the receiving device -- whether or not, or how well, it can capture the signal as the eye pattern turns to mush.

Having said that, 900 feet over RG59 is bound to be trouble, even over the premium shielded variants like Belden 1505a. For 720p, that's rated for maybe 200 feet. Your footage may vary (-:

RG6 will be better, but again, shielding matters so you're better off with 1694a or equivalent. Even then, I wouldn't gamble on 900' without testing it in place.