You seem to have hit the electric dilema.
Now days, most of the time the answer to batter questions or problems is Li-poly. With the small brushed motors, this creates a problem. Li-poly doesn't give you much choice in voltage, usually 7v or 10v for our applications. The problem is that at 7v the motor usually doesn't creat enough power to be of much use, and at 10v power is outstanding but motor live is cut down to as little as 5 or 6 flights.
OTOH you could go nicad or nimh, I personally prefer nimh because they are generally lighter, but still much heavier than li-poly. Adding an extra cell to your batt pack will give you more power, but you'll loose some motor life.
You could use 720mah packs and generally get 6-8 minutes with an 8cell pack, but your motor will be at the limit of these cells for amp draw.
You could use some 8cell HEcell 1100mah packs, they will give you approximately 10-12minutes.
There is also the 8cell KAN 950mah packs. These packs will handle higher amp draw and give more usable power than the HEcells, expect an extra minute or 2, but they are also about 1oz heavier.
KAN also has their 8cell 650mah pack that is about 2oz lighter than their 950 packs. I don't have any of these packs so I don't know about performance, but they are supposed to be great up to 10-12amps which is more than enough for your motor.

Just keep in mind that a heavier pack will change the way your plane flies.
I hope this helped.

Jason