The actual ignition of the SSME's is taken care of by devices very similar to spark plugs, located in each of the turbopump preburner combustion chambers, and in the main combustion chamber, of each engine. Click here for detailed explanation of the nine levels. One source says it's faster and cheaper, another that it's slower. Answer Ion propulsion systems enable very high speeds, but accelerate very slowly over long periods of time to attain those speeds. They are suitable for relatively small spacecraft that have a lot of time to get where they're going.
Cost is one of the most important considerations in any space transportation system.
Because ion propulsion systems are much more efficient in their use of propellant, they lend themselves to smaller spacecraft and reduce the amount of mass that must be launched into space. Both effects tend to yield lower costs than other alternatives. All rockets develop thrust by expelling matter, resulting in acceleration of the rocket in the opposite direction, as governed by Newton's laws of motion. The greater the mass of matter expelled, and the faster it is expelled, the greater will be the thrust generated. And the greater the thrust generated, relative to the mass of the rocket, the greater will be the acceleration of the rocket.