It’s going to be application dependent. If you’re looking for a bench rifle for ringing steel out at 600+ during daylight hours, you likely can get and will benefit from more magnification. Make sure to match quality with magnification. Cheap 20+ mag isn’t going to do much. For hunting, if you are competent out past 600, which few hunters could claim to be, remember that the perfect situation (rest, eye relief, eye alignment, daylight) will be harder to achieve. Higher magnification is going to amplify slight changes to your shooting form or imperfect situation (which, again 600+ you’re probably in close to perfect situation anyways), so modest magnification with high quality glass will buy you some extra grace.
If you’re hunting at 600+, you’re likely less interested in close-range encounters and will find that the first vs. second plane debate is dependent more on your style of hunting and location/opportunities. You will likely be better served to try and set up a rifle for either long-range hunting, or close range and not try to have a do it all. You’ll end up compromising on both ends and be somewhere in no man’s land, with a rifle sub-par for both.
What rifle are you setting up, out of curiosity?
Good luck! Looking forward to hearing what you decide!