Well we have both and each have it advantages and disadvantages.
Plug in~ Well if you have access to electricity, this works great! You can be pretty confident that if you have your charger plugged in, it should be sending a current through your fence. We have a one of these chargers in our barn and it powers the fence around the barn. They tend to be cheaper to buy then solar or battery charger.
Solar~ We have one of these as well. It is great when it was new and worked well. Right now it is not sending out the best charge, only 1000 volts or so, it should be 4000 or 5000 volts. When the charger was new and worked well, we could pasture the boys in fields that there was no source of power or if we did not want to bother the closest home owner with plugging in the electric charger. We use step in stakes and poly wire to fence our pastures. So easy and movable. We have changed the rechargeable battery and it still is not sending out a strong charge.
Now what I would recommend for you? Well it would depend on where your going to use it. If it is around a power source, like a house or a barn and it is going to be permanent fence, maybe go with a electric charger. If you already have an electric charger, and you plan on moving your horses from one pasture to another and have to run a long power cord or there is no source of power, go with a good quality solar charger. I have said my solar charger is not working the greatest, but it is several years old.
Either way, I don't think you can go wrong with either an electric or solar charger.