I'm not sure what plan to choose.

This is my situation...
At the moment I'm on an Optus corporate plan from AAGEEESS ago that has 'yes'time and good old 'yes'weekend with 18c/30 seconds peak rate and 9c/30 secs off-peak rate.
I only got a 3G phone a few months ago, and for some reason since I activated 3G services on my SIM, I haven't been charged for WAP? I don't know if it's because it's a corporate plan that I don't get charged for it or if it's some kind of remarkable billing error.
My phone bill averages to about $30-$40 a month though it can fluctuate even higher if I have SMSed a fair amount. Most of my talk time is during 'yes'time.
Before these rates came out I was inclined to buy the phone pre-paid/outright and continue using my current plan. I was prepared to pay around $800 for the 16GB anyway, thus the price is reasonable.
But now, I'm tempted to go on the $49 cap, seeing as though I pay around that much on my current plan anyway, AND it comes with data. HOWEVER I don't know if I will use the data. AND if I continue to get free data as per above, I don't need the new plan.
So what should I do...hmmm
EDIT:
Now if Optus came out with add-on data plans that would be perfect...
Guess I'll just wait for the cheapest pre-paid iPhone, though I suspect they should all be the same.