Ah makes all the difference. Think of it as how long a battery will last.
Okay, it varies a lot with makes etc but with mine a 36Ah will last about 10-15 miles happily. That's the same Ah as the C5 originally came with. 60Ah will do about 30 miles and my big 100Ah has done nearly 40 miles and still had charge left. Batteries are tricky though as they also fall into different catagories so I'll try and keep it simple (and others please correct/add as appropriate!)
Lead Acid 12vYour classic car battery. The problem is they're designed to kick out a lot of power for a short amount of time and not discharge fully, so even a 36Ah one of these will only do a few miles.
Lead Acid 12v Deep DischargeThis is the usual leisure battery. Does same as above except it's designed to release the electricity slowly over a long period of time AND use all it's power before it needs recharging. That's my 100Ah. Drawback is they're bloody big and heavy, but the cheapest decent distance option. Unusual to find a 36Ah one of these, most are 50+, but will happily do 25 miles+
Scooter Batteries - Gel etcThese are the favoured ones now. They're tiny compared to deep discharge lead acid's, so a LOT lighter, do a similar amount of distance, so a 36Ah will do around 15 miles+ BUT they're a fortune brand new. You're talking £80-£150 minimum for a decent one... BUT they do come up for sale second hand. If a scooter's been stolen/recovered, written off, then people sell the batteries but you've really got to be careful they're not just selling them because they're dead...
ChargingThis is the flip side of it. Lead acid's will take a standard cheap-as-it-comes car charger happily... scooter's generally won't. It needs to be one designed for gel. I picked up a nice charger last week from Halfords for £40 that does gel, but the charger needs to be a consideration when buying batteries on a budget, because even though you might find a pair of scooter batteries for (let's say) £30, you then need to add the cost of a charger for them on top of that!
Understand now?