suppose i need a 2s lipo with 15 amp discharge current

so which should i use 
2s 20c 750ma

or

2s 15c  1000ma

both combinations can meet your requirement

You can choose anyone as both of them are compatible with ur requirements. However, the 2s 15c 1000mah will give you a greater flight time than the 2s 20c 750mah.

It is better to use a battery whose discharge capacity is at least 20% more than the maximum load current for safety sake.

Yes I missed that part but exactly as Rao uncle has said, choose a battery with about 15-20% greater discharge capacity than what is required.