If you connect positive power nodes to electric motors their power increases, much more than for combustion engines. If I remember correctly, adding a positive power node (which costs 5 power) to any combustion engine gives you 1 extra power, while for an electric engine it depends on the model but is it always much more than 1. Example: the earliest electric engine gives +11 power per positive power node.

Combustion engines provide better stats if you do not use power nodes, however they are surpassed by electric motors when you start connecting them to positive power nodes.

I usually prefer to convey power to weapons, so I do not use too many electric engines. I go for the better combustion engines on my mechs with the most advanced power generators (termonuclear or nuclear reactors) and use the extra power on railguns or laser weapons. For my mechs with basic weapons (miniguns or rockets) which do not require much power, I have one or two electric engines to get extra power without using the better combustion engines.

Not sure about the 1rst gen electric motor,

3rd gen ICE motor have a weight of 4, 80 power,
with +3 red wire it totals 83 power (79 net) and -16 energy

1rst gen electric motor have a weight of 10, 55 power
with +3 red wire they go up to 88 power (78 net) and -26 energy

They consume more power, have 1 less max weight, and provide less speed than the combustion motor unlocked earlier.
They are faster and cheaper to produce though... probably the main point.

DIdn't unlocked the 2nd gen electric motor yet but i bet they'll be superior in strength, maybe speed too.