I can only comment on petrol 1.4 2010 EX-T - which my wife uses very much as a second car for around town. The truth is that the Jazz is fantastic - and does exactly what it's designed for - running round town and to the shops. But if that is not the only thing you want remember when downsizing to be honest about what are you actually going to use it for - be very certain. The Jazz is not the master of all jobs - I have driven it on several 600 mile trips, across very smooth european motorways and when doing so would recommend:
a. Taking neurofen to stave off the headache - yes it really will give you one from the noise, excessively hard ride and in the 1.4 heavily revving engine - suffering from the lack of 6th gear when doing 70 - and see fuel consumption plummet to 28 mpg! My 3 litre BMW 530 D Msport estate (2 tonnes) does 43 mpg at 120 mph! The Jazz drinks fuel at 70 mph - but does great round town (51mpg)
b. Carry a soft cushion - because as described frequently in reviews the compact drivers / passengers seats (made that way to give leg room in the back) are like perching on a sack of cement and are to hard, too small and when combined with overly stiff suspension lead to.....
c. Consider taking quells to avoid seasickness - as with the body roll, lurching suspension and unbelievably harsh brakes. The brakes are designed to allow grandma to break at the last moment before hitting the pedestrian and are simply binary - they are either - off or throwing you through the windscreen - and I mean throw. They are undoubtedly the most fearsome brakes I've ever experienced in any car - and my other ride is an Aston Martin 430 bhp vantage, and it doen't brake like the Jazz!!
In summary - despite what I've said the Jazz is wonderful for occasional use , or regular use round town - but it is simply dreadful on the open road, and if you are driving a larger and smoother car now you will be disappointed if used in that role - so really thats the break point - round town roller skate with lurching ride and the harshest brakes on earth, or if you want to go long ridees occasionally look for a cheaper, more refined but possibly less reliable alternative if you want to get out on the open road.
My wife loves her Jazz, Mrs Clarkson loved it, but even my wife shuns it for a 50 mile trip round the M25!