I would recommend that you test drive both if you can. Running costs aside, they are different to drive, and have different appeal.
The 400h is more powerful and is faster, but at the same time makes for a more relaxed drive in my opinion. Whilst the 300/350 have a conventional auto gearbox, the 400h relies on a CVT/stepless gearbox. It is quieter than the petrol model at lower speed thanks to the hybrid system, but on the other hand screams at you when you put your foot down whilst the CVT "catches up". Also, the two behave slightly differently on windy roads as the 400h is heavier, although i couldn't say which is best.
As pointed out by others, the 400h was never available with air suspension (I think this was because of the space taken by the hybrid system), and apart from the different gearbox other differences are minimal. I seem to recall that the 300 has conventional rear lights, whereas the 400h has LEDs.