I'm trying to decide how "much" scope to put on the SPR I'm putting together. If you had your choice of the three, which one would you choose.  Assume all are from the same maker and the objectives would be the same(either 40 or 50). The target engagement distances would be out to 600 yards.  The three choices I have narrowed it down to are:
4.5-14x40
  4-16x40
6.5-20x40

My main concerns are the upper and lower ranges.  Is 6.5 too much power for 100 yards and closer and is 14 enough for 500-600 yards.
Thanks in advance.

VSP

I'm assuming that this is for target shooting and plinking right?

I have scopes in the first and last powers you state - the 4.5-14 is a 56mm on my M1A or Remington 700 VSS. The 6.5-20x is on my .22-250 varmint/target rig. The extra 6x worth of power is really helpful at longer distances and for getting it right even as close as 100 yards.

Typically when hunting for my target I'll go to the low end of the zoom and then once found I'll crank it up to the high end - even up close.

I do find that 5x is about all the more power you want to try to shoot standing. Above that power and the target is wandering around so much it's hard to see and hold a shot - magnifying your wobble by 6.5 times makes shooting free standing very difficult. From a bench, bi-pod, or even a steady against a tree - no problem.

14x might be enough power at 600 yards depending on what you're shooting. I shoot 8" paper plates and a 18" rifle gong and need every bit of magnification I can get. You'll also find that shooting at those ranges with a tiny .223 you'll have to do a very good job of range finding as the drop is amazing way out there.

Getting a 30 mm tube scope makes sense - or at least one with lots of adjustment - 60 or 70 minutes of angle.

The first two choices have very little difference.  The third one's power goes so high I worry about the exit pupil diameter and personally would go with the 50 front end at those powers.  I'd go with the second as it simple is the same as the first but with broader range.  Make the third a x50, I would take that if you are going really long distance, and if you are mind what Paul said and make sure it's a 30mm tube cause at those distances you are going to need a lot of adjustment room.

3.5-10x40mm LR M1 Tactical. Illuminated Mil-Dot (30mm main tube)
Or without Illumination for the wt. factor
you can always put a Doctor on top
make sure you get good rings
Just leave it on 8X- imo

The smalller the target, the higher the magnification needed to identify and hit it.  The SOCOM SPR's are using the 3.5-10 LR M3 or th new TS-30 A2?, which is even lower powered, like 2.5-8 or so.  These are plenty to hit man sized targets out to 600m.

For punching paper, the more the merrier...  4.5-14 x 40 is a nice compromise in power, size and weight.  If you will be routinely shooting from a bench or bipod at 600 plus, go with the 6.5-20.