It seems the battery I ordered from banggood at the time of buying my GEPRC Crocodile Baby 4in LR has been lost and they’re in the process of refunding me, so I’m in the market for at least one more. Not that I’ve flown a whole charge on the one battery I do have yet. 
Anyway, I know I need a 4S with XT30 connector but I’m slightly lost on the rate and capacity. I understand basically what they both are, and the specs say 650 - 1300mAh is suitable, and I did read Oscar Liang’s assessment which, frankly, was far more effort than I was looking to expend on simply picking a battery. 
So, fundamentally, what’s the difference between a 650mAh, 75C and a 1300mAh, 120C? And would something in between be perfectly acceptable? More to the point, will a noob like me even notice the difference?
The one I have already is 650mAh, 75C.
I suspect the practical answer is, “Whatever you can find in stock.”
Its the weight. Your quad will have a thrust to weight ratio. Higher the better for performance but short flight time. Lower for slow hovering longer flight time.
I’d say 850 to 1100, depending what you are using it for.
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As with many things, it’s a balancing act.
More mAh means longer flight time, but more weight so the extra flight time isn’t as long as you may expect i.e. double the mAh doesn’t translate to twice the flight time.
The added weight can also affect the speed and handling of the drone, especially if taken to the extreme.
On my 4" drone ( albeit with 3.5" tri-blade props in place of the original 4" two blade ones ) I generally fly with either 650 or 850mAh batteries - though it flies quite well with 450 or 550mAh the flight times are shorter so I generally keep those batteries for my 2.5" drone.
My main batteries that I use with my LR Drone are 850 mah tattu’s or 1100 mah 4s gnb’s.
The gnbs are pretty small and give around 8/10 min flights if I go steady with them.
I’d put a stock notification on them as they literally fly out when they come back in stock