I currently own a 57 plate Focus ST2 and have previously owned a 53 plate WRX STI and both cars are great. The Focus is definately a better car to live with on a day to day basis, but they are nowhere near the performance of the STI. It depends what you want out of it.
Fuel costs wise i'm not seeing a lot of difference between the two, but i do have a heavy right foot
. If you drive "normally" then the ST2 will perform better than the STI because no matter how i drove the STI it was awful on fuel, but mine did have the TSL 333 pack, however don't expect anything above mid 25's mpg wise with the focus no matter how you drive it. You will only see above 25 mpg on a long run cruising in 6th doing 70mph "ish".
I know the subaru "burble" is legendary and i loved the sound the STI made, but i personally prefer the noise the focus ST makes, that V5 makes a lovely noise, especially with my recently acquired mountune cat-back system
- i know this is completely down to personal preference.
As you mentioned in a previous post the mountune ST260 upgrade can be had for about £1200 (burns in congleton now offer this), and from comments on the STOC people who have done this say it does completely change the car and it becomes a bit of animal - this is next on my list of mods
The performance of the ST is very good, and the torque it produces is quite superb - the handling is very good for a powerful FWD car, but i haven't been in a Fiesta ST so i can't make a comparison.
Running costs can't really be compared, the WRX STI was very expensive in every aspect, fuel, tyres and especially brakes. When my STI needed front discs and pads i was quoted £400+ VAT from the main dealers, i know there are plenty of after market options available but all decent one's from my experience weren't that much cheaper. Focus ST wise your only major running cost in comparison to a WRX STI is fuel, everything else on a focus is cheaper, end of story. Opting for a WRX is pointless, yes they are 4WD but only produce around 225bhp (same as the focus ST) and as a previous poster mentioned you will just regret not going for a WRX STI. Yes there is always the option for modding a WRX but you can mod a focus as well.
I live in tunstall mate and i'm a regular visitor to OCUK headquarters
and if you ever want to meet up and have a go in the focus without having the watchful eye of a dealership sales man with you then i'm happy to come and meet up and take you for a spin.
PS - Gibbo if your reading this i have been very jealous for years of your lovely history of motors. - seeing you mustang outside the old OCUK at fenton always put a smile on my face