There is no reason really to not take the shorter barrel in your case besides preference. If it is a rifle that I am going to be humping around, you can bet I will go for a shorter tube every time. Bench guns, ELR etc, dedicated target rigs are another ball game...
I like my shorter barrels. Sure, you lose a little velocity, but it is nothing that really matters in most cases. With the range you stated, both barrel lengths will perform admirably. My favorite 308 I have ever owned had a 22 inch tube and it could reach any distance I was comfortable shooting.
My SPS shoots great (223, not 308) with the 20 inch barrel. HOWEVER, I did not like the Xmark trigger. Mine was not the "user adjustable" one though. It was fairly crisp, but I was never happy with the weight and overall feel of it. I cannot speak for the newer one though. You may have a different experience. After changing that out I am very happy with the rifle. Dropping in a new trigger, like a jewell or rifle basics, or tuned older rem or something, is pretty easy to do.
The hogue stock looks and feels great, BUT it is fairly flexible at the front and may contact the barrel when shooting off a bipod. I never really noted and major negative effects on my targets (but it bothered me and opened the channel a bit). This is easily fixed with some creative bedding and stiffening rods if you are feeling crafty, or just simply opening up the barrel channel up a bit like I did.
If I do my part and load well, it is very consistantly a 3/4 minute stick. I really like the 1/9 twist that has worked well on everything up to and including 69g smks--which I currently shoot.
**I am not sure of the twist on the varmint model, but I think it is a slower twist aimed at lighter bullets** May be worth considering if hunting big game or longer range targets.
Just about any time I go shooting for fun, this rifle comes along.
feel free to pm if you have any other questions