I spent ~3.5 weeks w/ Stray during a 6 month RTW trip and loved it.
I was a solo traveller so this was a great way to have people to hang out with the entire trip. Going through NZ alone might be tough just because it's so sparsly populated.
I did a post about my trip in case you wanted more info, I just didn't want to bog down the post by re-posting everything I said.
At the end of the Day I'd definitely go with Stray over Kiwi. It skews older and you get a little bit further off the beaten track, and in general you should get involved in a hop-on-hop-off bus if you have the money to do so.
[ Edit: Sorry, no promos please. ]