I second a variable 1-4x or anything fixed in that range for a traditional AR (a carry not benchrest rifle). If you benchrest it for 300, stronger is better but I have not done it.
For a 100 yard 22... Assuming you shoot supported, I'd go with a higher power scope, a 3-9x would be average, or a fixed power unit (save money to get better glass), depending on your eyesight and your resistance to visual fatigue.
For extra $$ you can have a scope with a red dot and a reticle. Leupold and Trijicon sell them. I bought mine from Leupold, and it gives you the best of both.