Its a difficult one.
I've currently got a 57 plate vRS and like you I kind of fell in love with it on test drive and didn't think a Golf GTI justfied it's extra cost and the Focus was a bit small and would cost more to run.
Previously I'd had a Mondeo ST TDCi and decided to go petrol because my mileage reduced substantially. I thoroughly enjoyed owning the Mondeo and whilst the diesel engine was a compromise over an ST220 it drove fantastically well for a big saloon car. Perhaps not in a thrilling way, but the turn in, grip and feel were great and it was only when I drove something else and it was sliding about all over my route home at lower speeds than I would carry through bends in the ST that I realised just how well engineered the chassis, suspension and steering were. Sure, it would understeer if you pushed it too hard but you could always feel it coming and lift off before it was too late. After 2 years I still enjoyed driving it and never tired of throwing it around my favourite country lanes.
With the vRS I took it easy for the first few weeks to run it in and then started to push it. The engine is a peach and if you drive it sensibly it will do 35mpg, drive it like a loon and it will drop to 25. The brakes are fantastic. The ride and handling aren't so good - the ride is quite choppy and the body control is not as good as the Mondeo. Whilst it will ultimately grip as hard and the Mondeo it doesn't quite inspire the same confidence or have the same feeling of stability and the whole thing feels a bit of a compromise. With other cars I've had things have got better as I've owned them for longer and had chance to explore them and with the vRS I feel I know more or less what it can do and rather than getting better I keep picking holes in things. Perhaps the Golf GTI is worth the extra - a proper hot hatch where the Octavia is a quick family car.
I think I'd swallow the extra cost and have Focus, especially in 08 spec.