Sunlight can work, but it is difficult to control. Tank lighting we can control accurately by comparison. Direct sunlight should not strike the tank, as it can heat the water.
I experimented with a 10g in front of a west facing window a few years ago, and had it running about a year. No light, no filter, just a heater. The problems (cons) were algae (even with blinds on the window trying to control this by not allowing direct sun to strike the tank), the plants all grew toward the back of the tank instead of upward which looked odd, and without a tank light you really could not make out much very clearly.
