Thats it in a nut Shell. Were as Balls may be a little bit more forgiving and a bit more hardier when it comes to humidity and temperature variations the Bloods will not be. That said as long as you do your homework as with any new snake and make sure you are equipped to keeping them at the correct Temp Humidity requirments you will have many happy years of Blood python keeping. you just need to be able to pick up on slight signs of concern alot quicker
As mentioned the 'aggressive' image they have recieved is totally out dated and captive bred youngsters, although maybe a little less tolorable to been messed around (then Balls) will generally be fine to handle and work with.
Lots of people move from Balls to Bloods as it seems a natural step for people who want to get into more exciting and challenging species. And theres no snake in my eyes more rewarding to keep then blood pythons