I own both; in terms of overall use, the Oppressor wins easily. Head to head the stromberg wins but in free roam the Oppressor is more deadly no matter if attacking or defending. The main limitation of the stromberg is against aircraft; it can't lock on to choppers or planes above it and it has a reduced lock on range compared to aircraft and other vehicles.

The Oppressor can gain significant altitude and go toe to toe with jets, and it's nearly impossible to hit with rockets unless they're from the stromberg/deluxo/ruiner. The Oppressor also had no problem catching up or escaping, while the stromberg has the same land limitations all ground vehicles face.

Near a body of water the stromberg is nearly indestructible and has great killing potential, but the problem is that it's very easy to avoid waterways. So that's almost all defense.

I love having both but the Oppressor is my main pick still, even if it's gotten boring to use.