Both of those, but also additional mass due to extra plumbing and such. It's definitely a thing we could do but it's not without it's problems. Also, Hall thrus

Both of those, but also additional mass due to extra plumbing and such. It's definitely a thing we could do but it's not without it's problems. Also, Hall thrusters don't tile well as they're annular/ring-shaped devices. There are linear thrusters which would tile better but they tend to work fairly poorly.