I have had both. They are both good in the rain.
Control may be better in the wet with the so3 due to the stiffer side wall. I didn't like the harsh ride and tramlining at the end of the tread life, but this is not part of your question.
The GS D3, ride better, last longer, cost less, have a softer sidewall, and have phenomenal anti hydroplaning. I believe they are lighter too.
The anti hydroplaning effect could give you a control advantage in extreme rain conditions.
I have not a control problem with either tires in the Subaru 17" optional tire size.
I am on my second set of GS D3 tires. I get about 20,000 miles of so3 and GSD3 tires.