I'll take this back to the original question.
I have yet to work with either one; however BT5R3 is more current than Blackbuntu 10.10. Although according to the website, a Blackbuntu 12.04 is in the making.
I would also like to know which one would be better to learn security testing on (I want to hack legally), and if switching back and forth between the two later on would be easy?
In my opinion, any OS is okay for pentesting, such as Windows, Mac OS and Linux. You need the skills and tools.
I think the most important is your skills and then the tools. Even you have a lot of tools such as those inside BackTrack, you do not have basic pentesting/hacking skills or knowledge, those tools are rubbish.
If there is no suitable tool for your pentesting/hacking, you can create one if you have skills or knowledge.
I don't believe so called "industry standard".
it is all about what colour do you want your toolbox to be.
That is all they are, a toolbox filled with tools, a true raftsman can carve a masterpiece from a rock with a spoon, but give a monkey a chisel and a hammer and you best get out the way. (which reminds me of this funnily enough https://www.youtube.com/watch?v=GhxqIITtTtU which sums up most Backtrack users
If you know what you are doing with the tools it doesnt matter, if you dont then none of the tool boxes are going to help anyways.
BT for me from way back when it was whoppix and iWHAX and looking forward to Kali which is coming soon.
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Backtrack - Giving machine guns to monkeys since 2006
Kali-Linux - Adding a grenade launcher to the machine guns since 2013