Teak is usually a golden to light brown with straight, open grain. It's naturally oily and has been traditionally used in things which are exposed to the elements and need to last a long time. Because of it's color and natural oils, it looks pretty nice, too. I built a bookshelf with it. It's not cheap.
I've never really messed with raw beech, but it's a fairly common wood for gunstocks, probably due to it's hardness, durability and availability. It doesn't have the long straight stripey look of teak, but it has a tight grain. What I've seen varies from super-plain to simple maple-y looking patterns. Most beech stocks seem to get finished pretty dark, probably because there is little in the wood to show off.
As far as oil finishes, I guess it depends on what you want to achieve. I've used Tung Oil (pure tung oil) on guitar necks and can tell you that it will leave a nice satin finish that wears well for it's intended purpose. I would have no problem with it on an SKS stock. Like any other oil finish, it's not going to wear or protect like an all-weather urethane, but it's usually sufficient. Tung oil won't darken the color of the wood much, either. Boiled linseed oil is probably more common for gunstocks, but I don't know that there's any advantage to using it. I think most teak oil is actually tung or linseed oil with some extra stuff put in to help protect your teak from the elements. I have only used it on some teak boat parts that I didn't handle much after refinishing.
