Who has shot both of them and can compare the two of them?  Living here in Iowa and wanting something more along the lines of a carbine length .50 for portability I'm leaning towards the .50 S-T because:

PROS:
1)  Less felt recoil than same rifle chambered in .50 BMG as I'd like an 18" barrel length.
2)  Uses less powder for reloading.
3)  Lower pressure and brass should (?) last longer than chambered in .50 BMG.
4)  Already have 4,000+ .50 bullets that can be used towards .50 S-T.
5)  Should work real well with .30 sabot rounds.
6)  Can rechamber .50 BMG to fire .50 S-T.
7)  Available in from .50 S-T from Serbu, Ferret and one other mfg I believe.

CONS:
1)  Brass is more difficult to find and more expensive than .50 BMG.  I've heard about $450/1,000.
2)  Just doesn't have the same visual effect of a .50 BMG loaded round when they're next to each other.
3)  I can't think of any others.

I know there is a couple guys with .50 S-T chambered rifles and I would like to hear what they have to say and give me your input as this will be a sizeable purchase for me and I want to do it right the first time.  Lastly, a .50 barrel can be rechambered from .50 BMG to .50 S-T as well as vice versa...correct?  Thanks for any and all input!

Thanks for your input guys as it's much appreciated.  The main reason for the Carbine length is for the ease in packing it around and could use it for hunting elk out on the property in Idaho if I wanted to.  I wouldn't load the M48 brass to BMG pressures and don't need BMG velocities out of the 18" barrel as it's just meant for shooting out to 500 yards or so anyway.  I'd remove the muzzle brake before firing any sabots which would only be shot for gee whiz or for inexpensive plinking for people so they could say that they shot a .50 (in theory).  I never even thought about the option of getting two barrels for a Serbu (BMG & S-T)...that would be something I could check some more on.

My biggest concern is the availability of the M48 brass as you don't see it as often as the BMG brass.  I've often thought about the possibility of forming M48 brass from BMG brass but don't know how feasible it would be.  

Thanks for the link to your range report MrBen but unfortunately it doesn't work on this computer...I'll try it again at home.