Hi,
The main difference difference between both companies is that Impressions is a property management company, and Ray Whites is a real estate agent.
It's pretty common for agencies to charge a one off letting fee, which is usually one week's rent plus GST. Sometimes, when the apartments have been vacant for too long and they're desperate to get a tenant in, they'll offer a week's rent for free, lower the rent or even scrap the letting fee, so keep an eye out. There's no harm in trying to negotiate with them as well 
As far as using an agency being an advantage or not, I think it really depends on how quick you want to move into a place. Our first apartment was managed by Impressions Real Estate, and they were ok, just make sure you're aware of what you should and shouldn't be paying for as a tenant, which I'm sure you are 
Jen
03.12.10: 5 years in New Zealand
14.06.10: NZ Citizenship Grant letter received
16.08.10: NZ Citizenship ceremony