I have both so I'll put it plain words. I prefer the xp90 vitz over the first gen swift. There's only one category where the swift wins me over and that's the m13 engine ( I love the sound that engine makes when you push it ).
Build quality, both of them are small eco cars. The built quality isn't something over the moon but put side by side, the vitz is better and you'll notice it when you look at the small things, the seams etc. Both car's developed rattles but that's normal due to our road conditions. I however have done sound deadening in both cars so cabin noise isnt an issue in either
The seating position in the vitz is better. The seats and the leg space is preferred too. The suspension is better too. I've driven time and time again in both cars to Lahore and don't feel tired driving the vitz as compared to the swift. I have lower back problems and the swift is just an overkill for me.
These are my opinions. Somebody might have a different opinion. So really, it's upto you to decide. Test drive both cars, see which one you prefer.