Hey alpsy, you're getting some good answers here mate, beginner's luck eh?
The most expensive concrete I have ever used (all 45cuM of it) was a white quartz type aggregaten and fines using white cement, all trucked up from south. The job was all polished afterward.
White cement would not be an awful lot dearer than gray, and would definitely be different, though it finishes up light gray not white because of the composition of the rest of the mix. Imagine the old white cement terrazzo mixes you used to see.
It would make a nice main floor with a darker border as has been suggested.
I agree with a Burnsey and Tubby2, stencil is passe', one off architect design jobs don't consider it any more.
I wouldn't polish an open area like yours though. I'd go for a good old fashioned exposed aggregate. No bigger than 10mm.
On another part of the white cement job we laid into the top of the wet surface different colours of smooth river pebbles. We then ground and polished the floor. They looked quite effective and set off the standard mix. Something like a few hand fulls of larger dia. smooth pebbles could set off a finer exposed aggregate quite nicely.
In Darwin we pay through the nose for stuff from south, but we have a few small local importers bringing in some nice stuff from Indonesia, like these graded river pebbles, and lava rock slabs.
There is no end to the decorative finishes for concrete, but I'd hate to be parking a car on something really exotic.
If you are doing it to make a feature, make absolutely sure it doesn't crack! People expect and don't notice cracks in normal concrete, but if you have gone to a lot of trouble then any sort of a crack will stick out like dogs balls.
On an area your size, it will cost stuff all extra to make it a bit thicker and use a thicker mesh than standard. Don't let the concretors add extra water to make it easier to lay. Pour it at the correct slump. Full stop.
Just as an aside, the guys that did the last lot of exposed aggregate for me, surprised me by using sugar as the surface retardant. I have always used a retardant supplied specially for the job. They used a couple of kilos of Home Brand sugar in say 10-20 liters of water and laid it on with a hand pumped garden spray.
They also used a 3000psi water blaster. Always with a large fan well away (like 1200) from the surface, but they had the power and the volume of water to hit it and finish it off at the optimum time. No p--ssing about with tiny Karchers or garden hoses. They did a twenty odd meter long driveway with a waving serpent like contrasting spoon drain down the middle. Totally consistent with no variation from pour to pour.
Cheers
Bill
