MapQuest says travel time is the same either way. Speed going through the park is slower but the distance is 16 miles less. If you're headed back to Houston after the stay I'd go down on 385 and back on 118 just to see both roads. Going back to Houston it's just slightly shorter to go up 118 from Study Butte plus you'll just feel like you're making no progress going through the park at 45 mph.
Coming from Rockport I'd take US90 out to the park rather than I10. Seminole Canyon State Park (after Del Rio) is a good place to camp (336 miles from Rockport if you bypass San Antonio on Tx173).