I think you also need to consider the purpose of having a snake. If you want one to curl up or hang around you neck calmly while you watch a movie, a rainbow wouldn't be a good choice. Mine are always "on the go" while being held. None of them just sit there. They are not too likely to be cruising around their enclosure during the day. A rainbow will likely cost 2-3 times more as far as purchase cost, and will eat like every meal is it's last. Balls are notorius for going off food for months at times. If you choose a ball, only buy a captive breed animal that is plump and has no external parasites or stuck shed skin on it.
The housing situation differs greatly. Balls are relatively easy to keep and are a good choice for a new to snakes person. Rainbows require much more specific and consistant environment. So the way you intially set one up will be somewhat different. As an adult the Rainbow will be longer, but about the same girth-maybe a bit narrower.
Do lots of research first, talk to lots of people, and remember purchasing a snake requires a long commitment.
Linda
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"There is a principle which is a bar against all information, which is proof against all arguments and which cannot fail to keep a man in everlasting ignorance- that principle is contempt prior to investigation." Herbert Spencer