1) fuel efficiency on petrol.
The newer one is more fuel efficient because an engine with better technology and a much better transmission i.e. CVT BUT, if fuel efficiency is really that imp for you then find an older vitz with manual tranny and it would make the biggest difference. Plus if you get unlucky and the engine faces some major issue then the one's for older vitz are available dirt cheap.
The 1SZ-FE is 1.0 L (997 cc). Bore is 69 mm and stroke is 66.7 mm, with a compression ratio of 10.0:1. Output is 51.5 kW (69 hp) at 6000 rpm with 95 Nm of torque at 4000 rpm.
The 1KR-FE is a 1.0 L (996 cc) version built in Japan and Poland. Bore is 71 mm and stroke is 84 mm, with a compression ratio of 10.5:1. Output is 67 PS (50 kW) at 6000 rpm with 67.07 lb?ft (91 N?m) of torque at 4800 rpm or 71 PS (53 kW) at 6000 rpm with (94 N?m) of torque at 3600 rpm
2) general maintenance like oil change etc
All depends on your priorities, if you go for original toyota parts then the difference would not be that great but if you are ok with the local parts then the one for older version would be cheaper. For oils, both cars have almost the same lubricants except the CVT oil for the newer model which is a one time expense btw so shouldn't be a really big issue
3) spare parts cost and availability - specially side mirrors
New model has more expensive parts
4) drive comfort
Newer is more comfortable
5) cabin space/capacity
Newer is spacious
6) pick and top speed
Both cars with 1 litre engine are under powered, so pick and top speed are not a factor that should even be discussed for them. You can although go for the versions with bigger engines as they are ok
Hope that helps