We have both a 2003 octavia and superb. They are both nice cars, the octavia is an ambiante (lower spec) and the superb is the middle spec, I think. Both were bought second hand, so no experiance of buying new. They are both good cars, and both 1.9 tdi. The superb uses more fuel, 40somethingish MPG in town, and 50somethingish MPG on long journeys. It is the 130 horsepower option though, and has a nice turn of power for overtaking etc. It is noticably roomier (we got it to put three car seats in the back, which just about fit, but it's hard to reach the kids from the front seat, as the car is long, this might be a good or bad thing), it feels heavier, and it's better at sticking to the road. It also has things like fog lights and air con (I don't pay too much attention to this stuff until I have to use it). The drivers seat feels a bit low, but comfortable. We had a look at the new superb, and without driving it, it does look nice. There's a complicated set up with the boot so that it works as both a hatchback and a saloon, don't know how that would stand up to use, but it looks nifty.
The octavia is, I think, 60 or 70 horsepower, it feels much lighter, and has a bit less grip on the road, this is not a problem so long as you know this, and drive accordingly. The octavia brakes very sharply (I have talked to a few people who drive them, and this seems to be common). The drivers seat is quite upright (which I find comfortable for driving) and is comfortable to sit in. The big big plus is that it gets 50somthing MPG around town, and up to 70MPG on long journeys.
Both cars have so far proved reliable and comfortable to drive.
Dunno if that was the kind of comparison you wanted, I don't think I've driven any other cars in the same class as a superb!