RG6 vs RG59 HD-SDI cables for live stream
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.