Not giving us alot to work with. Most rulesets claim to be able to do skirmishes up to large battles, but the truth is that most rarely do both.
Size. How many troops and vehicles do you want per side?
Complexity, it's very hard to have both.
You were at least able to specify individual basing. Thus, Future War commander might not be what you are looking for and Alien Squad leader would also not work. If you want to try those games, you would want to make movement tray bases for your figs or use one fig to represent a squad of troopers
Assuming that between big and small means 30-50 figs and a few vehicles, 5150, or Tomorrow's war. Tomorrow's war would be more complex and more detailed. 5150 would be easier but with less detail.
I would not use Stargrunt as while not being to difficult, it's definitely not easy.
One other free set that's getting alot of play over at TMP is
FUBAR.
http://thegamesshed.wordpress.com/category/wargames-rules/sf-wargames-rules/ All the rules on one page so it's very streamlined, but there are alot of one page expansions. All free.