A few factors that seperate them. As you mentioned, the Astros offense, at least once Berkman and Bagwell get rolling, should score plenty of runs, while the LA offense is extremely week. However, the run support is partially offset by the ballpark factor, with juice box being an extreme hitters park, while Dodger Stadium is an extreme pitchers park. If a pitcher is off his game at all, they can well get crucified in Minute Maid, which makes Oswalt and Miller both scary at times.
Then you have the injury factor. Brown's past two years is well known, but Oswalt could well be equally risky. Keep in mind that he is tiny (6'0, 175 lbs), and pitching as many innings as he has at his young age and with his slight build could eventually cause burnout and possible injuries. Overall, Brown is a slightly greater injury risk.
Really it comes down to what you are looking for in terms of stats. I would think that for the balance of the year, Brown should have a better ERA and WHIP, while Oswalt will probably win more games because of the offensive support. Take Oswalt for wins or Brown for ERA/WHIP.