The big advantage is power to weight ratio, Li-pos pack a lot more power in a smaller and lighter space, and are almost as cheap as an SLA with far superior performance and probably cheaper than a good quality Ni-mh that generally have a capacity of 4200 Mah for a high discharge type which realistically are limited to about 35 amps.
5 amp hour 3 cell 2C Li-po is $28 Us Dollars , it can output 125 amps , an SLA looses a huge amount of capacity at anything over 1/2 C rate discharge, so your 5 AH SLA will give about 2 amp hours at 2C (10 amps) Li-pos will output nearly their whole nameplate capacity even at high C discharge, a 25C Lipo will probably still give 4 to 4.5 AH at 25C discharge.
The Lipo weighs 450 grams, the SLA weighs more than 4 times as much at 1900 grams and the Ni-MH around 700-1000 grams.
Nick