"To get out of gravity well thrust/weight is key" this isn't really true. TWR matters a lot for the first stage, but by the seccond stage, higher ISP and lower TWR is worth it as long as thrust is reasonable. ion thrusters are great for light unmanned probes but anything heavy needs higher thrust and hydralox is key for that. Ion propulsion is also pretty much worthless past jupiter since you can't get enough electricity.