I will second Timbercruiser. Scope quality is way more important than either magnification or objective lens size. Glass quality and coataings will more than make up for 10mm of objective glass and you will get more reliability, better tracking adjustments and a functionally better warranty to go along with your glass quality.

Also consider eye relief. Not only is it important for protection from magnum eyebrow, but a larger eyebox makes a scope easier to get lined up on for a quick shot. If I had a choice between a 4x with short and critical eye relief and a 6X with generous eye releif I would choose the big eye box every time.

Do consider weight and portability if you carry the rifle. If you hunt from a stand and all you will carry is the 300 yds from truck to blind..size won't affect you. But if you tote that rifle over a few mountains......

Here are my picks for a value in a scope

$180 Burris 3x9-40 FFII, $220 Nikon Monarch 3x9-40(if you can still find one), $350 Leupie 3x9-40 VXII $400 Ziess conquest . If you budget is less than $180 I would be careful and consider saving for a month or two to get to the FFII. If its over $400 your starting to get to place where there are lots nice scopes.

Another scope of note is Leupies 6x42 FXIII. Its bright,reliable and light, Pretty much because it saves a set of lenses that would be in a variable.. It has that big eyebox that make it easy to use up close and 6X can cover as far as most shots should go anyway. I just bought one of these last week and I already wish I had gotten one earlier.