You didn't mention what vehicle you're running. When Lipos first became affordable, most of them where 20-30c and they were definitely a step up from NiMH. And given the theoretical amp output of c is c x capacity, your 25c battery can put out more current than the 40c one.

Just be aware that brushed motors are less efficient than brushless and therefore will overheat quicker. It's quite common for people to have batteries that can run longer than the brushed motor can.