EnderW,
For distances out to 600m, both the SMK 175 and SMK 168 are great. Use whichever one is cheaper/more available/etc. If you are going to shoot further than that, the SMK 168s have problems but the SMK 175s are fine.
Scenars are extremely high quality bullets. The 155 Scenar is a very high performance bullet but it is quite specialized. It has a "fussy" sort of nose design, meaning that it might not shoot well in your rifle until you go through a *lot* of load testing and tuning that you probably have not had to with the SMKs (that is one of the really great things about SMKs, they are distinctly un-fussy bullets). The 155 Scenar also has an unusually long shank, which is not a bad thing but it does mean that unless your rifle's chamber has a fairly long throat (and I think Savage's .308 throats are pretty short), the seating position of the bullet will be suboptimal (you'll have to push the bullet quite deeply into the powder space). The 167 Scenar is more of a general purpose bullet, it will be less fussy than the 155 Scenar - it's more or less a Lapua equivalent of the SMK 168.
Berger makes some extremely high quality target bullets too, and recently they have been making some very "unfussy" bullet designs (I love the 155.5 Fullbore and the 155 Hybrids look very very good; the 185 BT is the standard FTR bullet). They are quite pricey. They are "better" bullets than the Sierra but quite honestly the Sierras (SMK 155, SMK 175 etc) shoot great out to 1000 yards and are capable of winning major matches, so you needn't feel at a disadvantage if you are shooting "just" Sierras.
As for velocities. Many match bullets will get extremely good accuracy at short range (300 and closer) with just about any muzzle velocity. Because of this you will frequently see extremely mild loads listed, with very good accuracy indicated.
Two problems with mild loads at distances greater than 600. One is that a slower bullet has a bit more wind deflection (though really not *that* much) than a faster bullet. The bigger problem is that mild loads tend to give less consistent muzzle velocity from one shot to the next. At longer range, this results in a difference in time of flight, which will show up as a vertical difference in the point of impact.
With long barrelled (30") target rifles it is typical to load 155 match bullets to 2925-2950fps ("normal, ordinary full power load"), 3000fps ("nice hot load"), or even 3050-3100fps ("very hot load; not necessarily unsafe if competently developed and tested").
If you are in the Toronto area and shooting at 300 yards (and well beyond!!!) interests you, contact the ORA and shoot with them. They have an excellent set of courses over the winter, if you're lucky there might even be a spot available (though they sell out quickly).