A friend of mine has a Rainbow, and since the water holds all the dust, it's a bit better if you have allergies. I've never heard any complaints from her about it.

Though I don't know if Rainbows have any attachments with rotating brushrolls. Robert_smith395 is right about how the Kirby pulls the dirt out from deep in the carpet. Having a brushroll helps a ton. Though I've found that since Kirbys have so much suction, they don't work very well on throw rugs. The hose with the small upholstery attachment works great, and also a Kirby Turbo Brush/Zipp Brush works wonders on pet hair, and would probably work good on throw rugs.

However I do disagree with one thing; IMHO it's not a pain to change to the hose, since all you do is lift the light hood, turn the belt lifter and the latch, then put the hose on and relatch it. I can do it in under 15 seconds, and it's never a pain. Though that's just my humble opinion.

Personally though, instead of buying the new Kirby, I'd look for a used one, or a refurbished older model. Another friend of mine bought a refurbished Classic III model for $300, and it was rebuilt, polished up and ready to go. It has a cloth bag, but you can easily convert one over to the HEPA bag setup. I've seen the conversion kits for around $100, and they include everything you need.

I also hope my post is helpful in some way.