I agree with everything 2 QK 4 U said.
5-disc is if you want to lock up your converter during a race for a couple extra MPH or tenths of a second at the track. Some guys go ahead and force the single-disc torque converters to lock-up but it's really not designed to handle that abuse and the clutch material will eventually fade and become useless. The 5-disc is strong enough to handle it.
So if you're serious about your track times and want to take advantage of running in Overdrive with a lock-up clutch then spend the extra for the 5-disc. If you're just going to the track for fun and would like to save the extra money, do the single disc.
I agree about the stall speed too, if you think you might end up with a bigger turbo go with a higher stall.