It is probably a little tight - you are relying heavily on the first flight arriving on time. I don't know what the INTL arrival is like at that terminal, not sure it if is better/worse than the TBIT. The fact you are not changing terminals is a plus.
I have travelled domestically in the US on UA. The terminal isn't too bad but the security line is always a killer. Do you need to get a boarding pass for the second flight or will you be issued one in Sydney?
To be safe, it is probably better to go for the next departure, even if it adds an hour or two. I would be stressing the whole first flight about it arriving on time.
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