Been to Big Bend at 5 times since the early 70's. Its worth the trip. Study Butte is the town at the Western entrance. I've camped in Chisos Basin and down at the campground by the river. Both were good. Spring...mid-March to late April...is my favorite time in the desert. Lots of cacti blooming and sweet smells. Fall can be good too, cooler weather. I'd avoid the park from June to mid-September unless you can get a campground in the Basin. Its hot. If you have a high clearance vehicle, there are some nice rocky/dirt roads to drive.

As for the Guadelupe Mountains, most people stay at the campground near El Capitan. They're missing some beautiful landscape on the back side of the park. Think you have to go to Carlsbad to get there, its a long way. But, there is a neat water fall with a pool you can play in and some very good camping, both in the official campground and along the road in. The area is remote, so bring your own supplies, including some extra gas.