The simple equation P(ower) = V(oltage) x I (current) tells it all, folks- look:
If a motor has a power output of, let´s say, 120 Watts (and 100% efficiency which no device ever has), this could be realized with a 12V- motor which, according to the formula above, has to draw 10 amps then. It could also be done with a 24V- motor which only needs 5 amps to output the same 120W! So the 24V-motor is the better choice because it draws less amperage. And as long as we stay below 48V, dc-voltage is rather easy to handle because we don`t need very sophisticated isolation, safety precautons and stuff like that apart from a good fusing.....
So IMHO it does make absolutly no sense to stay with 12V designs because they simply have to handle too much current and the more current you`ll have to deal with the more expensive (and dangerous!) it`ll get. Starting with cables, connectors, fuses, electronic components and so on. 24V= or even 36V= is much easier to handle and normally all electronic components can handle that voltage easily.
Always remember: Current is what heats up components, not voltage! Current is what makes you use thick and expensive cables, not voltage! Current is what generates high losses & problems everytime you want to regulate it, not voltage!
Surely there are more factors to be considered when selecting adequate motors for your project but generally people should be more afraid of working with a 12v-system than with a 24V-system. 12V has only been chosen by so many people because everybody nows it from his own car- stupid decision!
Besides all this I´d rather go for 24V wiper motors because they`re made to move huge wipers on trucks and thusfor normally have to be way more powerfull than their smaller 12v counterparts, to me that`s a no-brainer.....
PS. Look at the power increase on battery-powered tools during the last decade- nowadays it`s not difficult to find devices that work with 24v-batteries rather than 7.2V on which they started back in the old days..... why do you think all manufacturers did only increase voltage and NOT amperage?