If you are concerned about conserving energy, would you use a gas or electric fireplace just for ambiance?

Another first world problem. If you are going to conserve energy as your question states, then don’t consume just for effect.

Why not use wood? After all, the word ‘fireplace’ implies the burning of something – normally wood.

I have thought of this. We almost converted our fireplace to gas for convenience and safety. It gets cold in the den we have in our finished basement and we used to build fires but we just use a small space heater now. The trade off of cost for adding an insert to use it for both heat and ambience just was not worth it in the end. It would be one of those things you use for a while and then the novelty wears off if it’s just for ambience. (The kind of gas insert determines if it really provides heat or not) It’s basically decorative now but in an emergency we just burn wood if needed. If we were going to use gas for heat we’ll just turn the furnace up, it’s more efficient anyway.

If you are running air conditioning then using the devices will make them run more and you will be wasting energy.
If it is the winter and you need to heat the house anyway then it is almost a wash. The electric heat is as close to 100% efficient as you can get. Gas systems are 95–98% efficient now too so it does not matter.
Gas looks nicer than electric but the exposed flame is a little more dangerous.
Just don’t run them when you will be using your A/C . That is really a waste.

I would use neither if I cared about conservation of energy.