i had used both 1600 & 1800 GP ones. all i can say is, the 1600 ones seems to be a good deal than the 1800 ones, which does not really deliver the performance it promises. that said, i had also discovered that Sanyo 1600 Twincell ones seemed to perform better. however, it really depends on your needs. as for leaks, i had been using 08 nos. of GP 1600 ones for abt 8 mths now, no leaks detected. as for 1800 ones, been going for 4mths, still so so, but also no leaks. however, i might still go for Jap brand ones, as they seems to deliver the promises. i don't really go for the Sanyo 1850 ones as they are very rare in the market & they had been said to be 'remarked' from 1600 ones.
all in all, if price is yr main concern, then GP is the one. if its performance wise, then perhaps the Jap made ones might be better. i said 'might' as it depends on yr equipment & charger.