Voltage does not matter when buying batterys and is not a valid way to compair batterys.
Watt hours do. Thats Voltage*AH of a battery.
It should also be noted that SLA's AH rating is typicaly a 20hour discharge rating. Meaning its how many AH you get at 0.05C or discharging the battery over 20 hours. At 1 hour, Its about half that. Below 1 hour it does not reduce too much, But its something to heavily consider as most electric bikes don't have more then 1 hour runtime.
Lithium is really king for portable electric storage in volume and weight. Loses a little in peak current (for starter motors mainly) and bigtime in price however to SLA. Nimh is a balance beween the two in most respects.
Lipo (Not sure about some of the newer chemistrys, I think some are safer) can and have been recorded to catch fire when improperly charged, punctured, overheated, overdischarged, etc, And generaly have 2 safty circuits (One in the charger or device, one on the battery pack itself) to help prevent this... Of course, On some chinese batterys these circuits where known to fail, or the cell would itself internaly, Catching fire.
SLA can emit some hydrogen that is a danger.
Gasoline fumes are well a danger.
However: Theres a big diffrence beween gasoline fumes ignition range and hydrogen.
"The flammability limits based on the volume percent of hydrogen in air at 14.7 psia (1 atm, 101 kPa) are 4.0% to 75.0%"
"The flammability range of gasoline is between 1.4 and 7.6%"
"Hydrogen-air mixtures can ignite with very low energy input, 1/10th that required igniting a gasoline-air mixture. For reference, an invisible spark or a static spark from a person can cause ignition."
Also, you can't smell hydrogen. And if any battery is punctured it generaly ends up.. Badly. Having a gas tank punctured/leak is usally just a smelly mess.
Batterys are made of everything needed to complete a high energy reaction. Gasoline is only one element and needs a rather precise mix with oxygen to react.
Overall I think gasoline is actualy safer then large electric storage systems.
And gasoline has a very good safty ratio. Electric cars have been known to catch fire due to small mechanical defects. And generaly gasoline never 'explodes' like you see in the movies.