Tonight I tried out the Kill Team rules from Heralds of Ruin. I have to say, if you haven't checked them out, you really should. The rules are simple, easy to follow and all work to keep the game fairly quick-paced.
The teams are set up as small 250 point squads using adjusted kill team lists (basically codexes) for each army. Each list does an excellent job of maintaining the flavor of the army. I can not speak to overall balance of the lists, but we played a game pitting Dark Angels against Chaos Space Marines and I can say that these two lists seemed balanced.
The rules definitely keep the game fast-paced and are very easy to follow while making the actions of the models seem heroic. The game we played took just over an hour and that was including set-up, tear down, and having to refer to the rules. I think once you become familiar with the rules, games should be closer to 30-45 minutes.
So for those of you who want a shorter game, or perhaps just don't have the money to spend on larger armies, this is definitely worth a try. And incidentally, for those that are curious, my Angels won the day!