For my .375 project early next year, I want to use a QD type scope mount.

Leupold has their QD/post in 2-pc. with the levers, plus the QRW Weaver style with levers.
Weaver has the Grand Slam rings, which I used on my 700 Police, and I like them.

Any advantages to either system over the other?
1 or 2 pc. base make much difference?
Is it really necessary to have the 8-40 screws?

As strange as it may seem, I have been told by thoose who supposedly know, that the two-piece base is stronger than the one-piece type, although I don't know why. I have used both the Griffin & Howe and Paul Jaeger detachable side mounts on rifles up to .375 H&H and found them to be excellent in their ability to return to zero, and they handle the recoil. The same holds true for the German Suhler claw-type mounts, but these are more expensive, and the rings are pretty much individually fitted to each set of bases, so interchangeability costs a lot of dough!! Jaeger and claw mounts are available from New England Custom Guns.

Of the mounts you have listed I have had the Leupolds with the posts/levers on several rifles and they have done real well. I called Leupold and asked if the post/lever mounts were strong enough for hard kicking rifles. The fella I ralked to stated they were the strongest mounts they made. I have also had good luck with Warne QD mounts.

I have a set of Leupold QR mounts (the ones with the post/stud) on my .458 Win. Mag., and these have been on that rifle since it was new - nearly nine years. All I can say is, they've performed perfectly from the beginning, with no POI shift or any other sort of problems. They're very well made, and in my opinion the most underrated detachable mounts in the world. Truth be know, I like 'em better than Talley's, and ten times better than Leupold's QRW (made by Warne) mounts....

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I only use Talleys QD today...

I saw a fellow break off the ear on a set of Leupolds with his hands, probably the only one ever that broke, probably a bubble in the steel, but we all go by our experiences...Soooo

I would still pick the Leupold over the Warnes, which I have absolutly no use for and had bad experiences in the field with...

As to two piece bases being stronger than one piece, I disagree with that, but both are sufficiently strong so it is a non issue on a hunting rifle.

The two-piece is stronger than the one-piece only if there are only 3 screws holding the one-piece base to the rifle as opposed to the 4 screws that hold the two-piece, and this is usually the case.

Leupold has lately improved their QRW, to include a square recoil stop that fits in the cross slot. Do not get the older ones with the rounded shaft lying in the square cross slot.

The new Leupold QRW's are as good as anything, and a lot easier to come by than Talley.

The old Leupold QD/post is good, if left alone. If used to QD very often, wear would loosen them. I would use them with a 1.5-5X, or 2.5X scope, and only remove the scope if it were trashed by a mishap.

A Warne Maxima that fits the Weaver square cross slot steel base with a square recoil stop is great too. Yes, Warne makes the Leupolds too.

Choices:
Leupold "New QRW"
Warne Maxima
Talley

Take your pick with no worries.

This might be switching gears a bit, but, since this Browning Medallion .375 H&H will more of a 'I gotta have one' than a 'I'm going in there after that wounded buff' kind of rifle...

I've always wanted to get the Conetrol setup for one of my rifles, and I think this will be the one. Not exactly a QD set-up, but, they ARE nice.
The Custum bridge base and whatever height rings will work with the NIKON 2-7 Monarch scope.
This product uses all 4 holes and has a beautiful finish to match the gun.

On my BAR, I used Millett 'engraved' rings on their 1-piece base to go with the pseudo-engraving on these guns. It looks good and will be filled in with the Brownells 23K gold fill kit I bought for it.

http://www.conetrol.com/catalog_page.htm