AFAIK, there is no practical difference between the 700 SPS Varmint & SPS Tactical bbls, other than length. For the most part, you aren't giving up much in the way of velocity by going with the shorter tube. Both SPS models come with injection moulded plastic stocks. You'll need to upgrade to the more expensive VS, LTR or Police lines for aluminum pillar blocks and HS Precision kevlar/graphite/fiberglass stocks. The 1:12 twist stabilizes just about every bullet weight you'd normally fire out of a .308 Win and a faster twist really isn't necessary.
For new shooters and those on a budget, the SPS line offers you an affordable way to get into the tactical/precision game. When budget's allow, you can upgrade the stock, trigger and glass, etc...
For factory barrels, the Remington 700 .308 Win varmint/bull bbls I've shot have all turned in excellent accuracy, with almost all performing at least MOA or better with factory ammo.