We have a Stihl electric long reach, which is necessary on a deep hedge. 650W motor deals with everything we have cut, but the cable does get in the way, as with all electric garden equipment. Looking at the Stihl battery trimmer the motor wattage isn't stated, but the battery is quoted as a 75Wh and has a run time of 50 minutes, so the power of the motor is about 85W- pretty weedy. All our battery equipment has been bought for very small jobs because that's all they are capable of. You need to be aware that the batteries get hot during discharge and won't recharge immediately- we put ours in the fridge for 15 minutes first. The weight of the battery unit is only slightly less than the electric because the former weight is quoted without the battery fitted.
So it's all down the what usage it will get. There are manufacturers now that have large tool ranges using the same battery (Einhell, Ryobi), so I don't see a problem replacing theirs.
I only buy Bosch power tools, but only Stihl garden equipment up to now. Having had a Stihl motor seize on me and being unable to get repair parts I may move away from them? Perhaps Bosch is the way to go if they have interchangeable batteries?