Re: Fixed power or not?
I use both fixed and variable scopes where they are appropriate. I also consider two distinct types of variable scopes. Those with first focal plane reticles, and those with second focal plane reticles.
The advantage of fixed power scopes are:
Usually lighter weight.
May be less expensive for equal quality and ruggedness.
Reticles for range finding or aiming offsets remain correct with any setting (assuming the manufacturer has built them correctly. Two of my favorites are the Burris 10x40 Mil dot (available in both mil and moa target knob clicks for under $250 and the Leupold 16x40 M1 Mk 4
in either mil dot or target dot. It's light and very rugged. About $1500.
The advantages of second focal plane variables are:
Gives the ability to adjust the magnification to match shooting conditions.
Changing magnification inherently also changes the apparent field of view and the image brightness. For a simple cross or dot reticle a second focal plane reticle has the advantage that apparent angular dimension of the reticle remains constant with changes in power setting. That also means that the any form of calibration from the length of a simply duplex reticle to a more complex pattern like a mil-dot reticle will only be correctly calibrated at one magnification setting. That can be useful by allowing the reicle to be calibrated in different units (Like Mils or MOA at different power settings if the zoom range allows it. Typically wrong magnification setting just causes missed shots.
The advantage of a first focal plane reticle is that it allows a range of magnificaton settings while retaining angular calibration of the reticle. It's at its best with various precision reticles calibrated in fixed angular units. Examples are the Horus Vision reticles calibrated in a mil grid, and some special purpose reticles calibrated for specific rifle/cartridge/bullet displaying bullet point of impact vs distance and a complex reticle of wind velocity vs target distance. It stays calibrated at any power setting. Some of the Springfield Armory scopes were set up for standard 7.62x51 and 5.56x45 ammo in M14 and AR-15 rifles. Those scopes don't correct for air density, but are simple and fast to use.
What's best? They are all good when used for what they're designed for. None of them will prevent you from hitting foot square steel at 1000 yards unless they're complete junk or broken. Hitting moa targets at 1000 will be limited primarily by your ability to judge crosswind velocity and correct for it. Ammo quality and your shooting ability will also be more important than scope selection.
Power selection depends several things. You need to consider all the sources of error you have in YOUR shooting. You want enough that the scope is not limiting your accuracy, but for a skilled shooter with good eyesight firing offhand in even a little wind iron sights (1x) can meet that criteria. You don't need to be able to count the points on antlers to be able to kill a deer reliably. That's what spotting scopes are for. More magnification just reduces the field of view and reduces target acquisition time. In my opinion magnifications over about 16x are suitable only for shooting at fixed (or slow) targets from a rest (bipod/bags/benchrest). High magnification scopes (>24x) are only really useful benchrest shooting or if you have really poor eyesight which require a lot of magnification to achieve adequate resolution. That is usually best accomplished with contact lenses or custom shooting glasses behind the scope.