I finally have some extra funds $$$ to change out the collapsible stock on my RR LAR8 .308.  My questions were:

1. If I decide to go with the UBR, will I have to buy a new buffer tube, etc?  Which one if so?

2. If I decide to go with the PRS, will I have to buy a new buffer tube, etc?  Which one if so?  I realize if I go with the PRS, some trimming needs to be done to the cheek piece riser.

Trying to decide which will be easier.  Changing out the stock on my AR15 with the UBR was a piece of cake.  Thanks for any help.

For me it would depend in barrel length.  I have a 20" one with a PRS.  If it was shorter I would go with the UBR out of your choices. I love the PRS on the heavy gun. It uses a regular A2 rifle buffer tube.

It's a 16" barrel.  I'm leaning towards the UBR - not really a long distance shooting rifle.  What else would I need to buy to assemble the stock to my rifle?

Here is a thread about some different stock installs on the RRA. The whole 308 thing is a little more complicated since there are so many different patterns out there.
http://www.ar15.com/archive/topic.html?b=2&f=38&t=196836

Thanks for the feedback.  

I'll do some further reading & investigating, then decide which stock is the easiest to work with.  Sometimes I wish I bought the Armalite AR10 for simplicity...

If you go with the PRS, Magpul has a version specifically for the 308/7.62 rifles.  The cheek weld is slightly smaller to work with the longer charging handles.

http://store.magpul.com/product/MAG308/181

The UBR comes with it's own  special buffer tube.  You will have to buy a new buffer however, since the UBR is designed for the AR-15.  Slash makes special carbine buffers for mounting AR-15 carbine stocks on AR-10/308 pattern rifles.

I'm leaning more towards the UBR still.  The rifle is kind of heavy as is.  I'll eventually change out to a FF rail.  

Is Slash the only company that makes buffer tubes that'll work for my application with the UBR?  I'd prefer not to pay $100++ for the tube/spring.  But if I have, to I will...

The UBR comes with it's own tube... It's the actual buffer/spring that needs to be swapped.  I'm unaware of any other options to Slash's buffers (not saying there aren't any), but that's who I went with.

Correct....proprietary tube.  I used a carbine (H) buffer and spring in mine...ran that lower on a 14.5" middy, an 18" intermediate and a 20" rifle....all ran great.

The UBR is a great stock...I used it on precision rifles and found the cheek weld to be sufficient...just a little heavy for my taste (again these were all 5.56 rifles, and I wanted them to be reasonably light).  Also gives you more versatility than the PRS, as it would work equally well for close quarters.

ETA - I have no idea about buffer tubes/springs for the UBR on an AR-10 (.308) platform.