@Vens,
If the price difference is not much Y trouble urself going down to Singapore to get it (there's also going to be a transportation cost!) and as U have highlighted, it'll be easier for warranty claims on the local sets.

Note: I've been using a NZ set since end Jul/early August last year and till now *touch wood* no problemos whatsoever

It's up to you mate. We would recommend that you get it here with Maxis since warranty is available here. Even a small loose button, they will change it for you with a new unit.

If you would get a Singtel unit, you would need to travel to S'pore to get it fixed and you need the receipt as prove that you purchase it from one of the local telco centres.

As for jailbreaking iPhones, they will not block the phone but if you take it back for warranty, they will reject your request to one to one exchange for a brand new set as they will not entertain a jailbreak iPhone.

Like tink and beavis said, both Maxis and Singtel iPhone units are officially unlocked. At the end of the day, it's your choice.