Haiku is more or less BeOS continued. After Be Inc. ceized existing (bought by Palm) BeOS became Zeta/Yellowtab. In immediate reaction upon that a few people began developing OpenBeOS, which became Haiku, a (fully?) BeOS compatible OS.
Where AROS aims to be a next-gen AmigaOS (with some classic AmigaOS compatibility as a nice extra), Haiku aims to be BeOS continued (subtle difference, but hey :-))
I don't think AROS and Haiku will be direct competitors, since they aim at different usergroups. However, like AROS, Haiku is well worth while the look.
I have used the original BeOS on my PowerMac some 10 years ago and I really liked the Amiga feel it gave me. I guess Haiku does pretty much the same.
Heh, I just thought about making a multiple-OS machine out of my PC. Lose Windows (or just a small partition) and add AROS, Ubuntu, Haiku and maybe one or 2 more OSes. How's that for diversity? :-D