What are the differences between these two besides the obvious? Does one have drawbacks over the other? Explain, please.

I would say the SPR one (first pic) works great on a bare flat top.

The one-piece LaRue mount positions the scope further forward, providing more eye relief.



Notice that the rear of the scope is forward of the charging handle.

Quoted: The one-piece LaRue mount positions the scope further forward, providing more eye relief.

Exactly

I love my SPR mount. :)

http://img24.exs.cx/img24/1633/samr0039af.jpg

One of the most important things to me is that the SPR mount has a downward cant of 10 MOA.  This means that my scope is given an "adjustment advantage" when being sighted-in at distance.  The downward cant gives me an extra 10 MOA offset before I have to move my scope's elevation adjustments to the extreme end of the adjustment range when I'm sighting-in at 500-600 yards.  In my mind more centered settings mean more accurate settings (not to mention giving my scope extra distance if required).

The one piece mount, quick release/return to zero capabilities, superior machining and finish all add up to one hell of a product - no matter what the price.  This is my second piece of LaRue gear (have an EOTech mount), and believe me, it's just the beginning of a beautiful relationship.

I use just the QD rings on my SR-25. Mine are the older style vertical split rings. The good news on those is that you don't have to lap them. I'm not sure how that will work with the newer, more conventional rings. In any event, their ability to return to zero borders on the unreal.

Some guys have issues with inadequate eye relief when using just rings without the forward offset of the full mount system. I've not had that problem with a Shepherd Scope, Schmidt and Bender, Leo 30MM variable, or Nightforce. I suspect that most of those problems could be solved with an A-2 stock.

Luck,

SD

That settles it! I'm getting the one piece! (when I get some more overtime...)

Beyond the reasons already posted, there is another factor.  The SPR mount is 1.50 from rail to center of rings for the 30mm version.  The rings are 1.27 as I recall for the 30mm.  I have the rings inbound along with the MK11 Mod 0 mount.  The difference in my opinion is that the  height on the rings seem better for laying behind the rifle and work out great as long as you don't need the built in MOA slant nor the extra flat top real estate for a long eye-relief scope.  I have the SPR mount and love it, but find while the height is acceptable especially for running and gunning, it seems higher than desired for the fixed position prone shots.  Great products either way.  I think there needs to be a middle ground of say 1.3-1.4 height but set-up with the extra reach and MOA advantage of the SPR mount.  One thing at a time though and the LaRue guys seem to be pushing at full capacity for good reason.
BT

Will the SPR mount allow you to see over the front sight post on a M4 with a 30mm leupold 1.5-5.

Where can I get me one of these LaRue SPR-15 thingys? :-)  That is just what I've been looking for.  Is there a web link to a vendor?

d.