I second the mentor idea....Learn, learn, learn....Find someone in your area that works with the species you're interested in and learn how to do things the right way from someone that knows...IMO, only then will you be qualified to "go it on your own"
All that being said, if the choices are between A. contortrix and C. atrox, it's A. contortrix all the way...These animals don't get large, and are rarely "aggressive"...they can be difficult to "hook", but overall make good, solid captives...
C. atrox get quite large, often have irascible attitudes and can pump ALOT of venom into you if you get too close...If you've never seen pics of a bad atrox bite, I'd urge you to view them before considering acquiring one...It's NOT pretty...alot of swelling, necrosis and other yucky stuff....While death is unlikely, limb/appendage morbidity is a DEFINITE possibility with this species....On a positive note, atrox usually eat well and present few husbandry problems to the responsible hot keeper...
Whichever animal you choose, understand that they can both make your life pretty darn bad if you make a mistake...therefore, train as much as you can before getting one of your own...
Good Luck...
-AzAtrox