You really don't need a 20x scope for what you are doing. I would just get something like a Vortex Viper PST 4-16x50 or something similar from bushnell's line. A 20x power scope in the price range you are looking will be pretty foggy/blurry and somewhat useless in the higher magnifications. Especially pass 20x. Spend your money on scope with better glass and not the magnification. Plus, you will have more reticle wiggle on target on the higher end magnifications, image will be darker, less range band etc. If you want something decent that is 20x+, you are looking close to $1000( Vortex Viper PST FFP, Busnell elite tactical, SWFA SS) If you want something good, you are looking close to $1500, Nightforce, US Optics, Leupold Mark 4, Swaroski, Vortex and SWFA's HD line of scopes, etc.
To be honest the Millet 4-16x50 TRS-1 will be perfectly fine for what you are doing, and you can save your self quiet a bit of money.
Also, if you probably want to get a scope that is in MOA, it is easier to range with.