The difference is quick charger is charging at higher current hence your batteries get hotter then the slow charger at lower current. Do not think quick charger is the best. As batteries with chemcials inside, have thermal limit. Might spoil batteries faster than a slow charger if limits are exceeded constantly. ie. shorten battery life in the long run.
Why GP seems not to be able to charge other batteries to full? [actually not true, i use to charge my other brand ones] The likely reason is they are optimize for GP batteries charge at certain level of current and when timer is up and it is off. If you use higher capacity batteries and put it there, of cos can't charge to full. The trick is to calculate how many hours to full charge and re-on the charger to charge your non-GP brands of batteries.
Alternatively, you can get a smart charger like the Maha MH-C401FS that I got recently. It will charge your batteries to the max capacity, regardless of the capacity of your battery. It also has a SLOW/FAST charge switch which is quite handy as well. Only gripe is the price of $50 for the charger alone, which is the same price as the Sanyo charger + 4x2300mAh batteries
Sanyo rechargable batteries are the best especially the industrial grade (same as consumer except the external cover offer better protection for rugged usage).
The most common cause of premature battery failure is overcharging. The type of chargers most likely to cause overcharging are the 5 or 8 hour so-called "rapid chargers". The problem with these chargers is that they really don't have a charge control mechanism. Most of them are simple designs which charge at their full charge rate for a fixed period of time, typically five or eight hours, and then shut off. In situation where charge cycle was interrupted part way through the charge. The charger is unplugged to see how warm the batteries feel or to use the electrical outlet for something else. Then the charger is plugged back in. This will cause a complete charge cycle to start again, even if the previous charge cycle was almost complete.
Suppose fully charged or partially charged batteries are put into the charger. The charger has no way to sense this, so it will give the batteries the full charge it was designed to deliver. It is not unusual to put partially charged batteries into a charger since it's pretty easy to mix batteries up and inadvertently put fully charged batteries into a charger. Do this enough times with one of these battery chargers and the capacity of the battery will start to drop.
The easiest way to avoid these scenarios is to use a smart charger, a charger with microprocessor control. A smart charger can determine when a battery is fully charged and then depending on its design, either shut off entirely or switch to trickle charge. Some like maha C204 has discharge-conditioning circuitry which is good for partial charged battery.
Most batteries are charged to about 85-97% of full cap when timer has clocked the "full charge". Some better bulit has "trickle" charge (to maintain charge for up to 24hrs) when others stop completely. Others may have leakage (back to the charger when leave in the charger) when charger stop charging. Also battery capacity will goes down with higher no. of charge.
For flash, I think the best is Ni-MH (Nickel Metal-Hydride) as they are suitable for high drain devices over a short period, good for use in heavy drain devices & more powerful than Ni-Cd and Alkaline batteries.