Flat trajectory is meaningless at 1000 yards, you must know the range and set sights for it.
The thing to look for is high ballistic coefficient and minimum wind drift.
Plug load data into your favorite ballistic program and see which is blown around the least. Hint, long range shooters are leaning towards heavier bullets than used to be considered standard. I thought a 175 gr .308 Win was a lot when I was shooting F-T/R regularly, but there are plenty of 185-208 gr bullets in use now.