The SiR isn't
that thirsty. Even my cousin's fully modified SiR drinks less than my 2.0. 7 km/l is in traffic, if you drive like a jackrabbit. If you drive sedately, and have no modifications, you should be able to get 9.
The SiR's fuel economy is about average for 1.6 liter cars of that time. It's just that the VTi is above average.
The SiR may be "stiff" by regular car standards, but it's not very stiff (softer than the Lynx RS, actually... which my baby can
sleep in), and it's liveable on a day-to-day basis. It's a good choice, but only if you want the extra performance. SiR values are starting to go down, finally, but they're still 100k more than comparable VTis. And, sadly, the only place you can use that extra power is on the highway. To access the SiR's extra overtaking power, you have to rev its nuts off... although, to be fair, its midrange is plenty strong enough.
If it's family use, go for the VTi. It costs nearly the same as the LXi, and it's almost as fuel efficient. It's got as much immediate pick-up as the SiR and it's still relatively fun to drive.
Of course, kung matic ang pinag-uusapan, throw fuel economy out the window. And I don't like buying 'matics that old, as you never know how well they've been cared for until they break down on you... better to buy a stick... saves money on gas, too.