@shortwavesurfer The propulsion is absolutely linear, the perk of an ion drive is that it's mostly electrical with minimal fuel consumption.
It's also something we're already using, the first one actually launched was in 1964, though for some reason we never stopped hyping it.
An ion engine would absolutely make the trip take longer as you'd have to wait for better transfer windows (9 months is the timeframe after we wait for a good transfer window), we'd have to wait even longer for one with an ion drive and it absolutely wouldn't be a shorter window.