It seems to me that the non-LED turn signals being sold to be mounted between the bumper bars are using very low candle power bulbs, as you would use for marker lights on a trailer. Useless during the day and not that great at night. I would not bother with them.
LED lights can be bright enough if enough individual LEDs are used in the light. They take up space and people want a small, unobtrusive light, so you can end up with too few LEDs in there.
You have more options with a 12 volt system to get brighter bulbs in place, if you want to stick to bulbs. I have 50 cp halogens in the back for stop/turn and they really light up, even during the day. An original 6 volt car will do better with LEDs, though, because high candlepower bulbs in 6 volts really take a lot of current.
I often hand signal at the same time as having the flashers going, especially when changing lanes to the left. It can't hurt. It can help get the attention of someone who is distracted by cell phone, radio, passenger, etc. while driving.
Let's be careful out there. Watch out for "the crazies"!