The Focus ST is an obvious Golf GTI competitor and I had the pleasure of test driving it 2 weeks ago - before deciding between that and the Golf.
The Focus has a lot going for it - it looks a bit more aggressive than the Golf and I personally like the engine sound better. My mate who was with me preferred the Golf's sound though. I also love how there is a distinct racing sound from the exhaust when you give it some revs and release the gas pedal (hard to explain and I don't know the correct word in English - maybe it's what's called backfire?). I don't think the Golf had that sound at least I didn't experience it during my test drives. Only in the DSG in S mode you have something a bit similar.
Overall the Focus' engine was brilliant and I could actually feel that it had more power and was 100 kgs lighter than the Golf.
The handling was a tad bit sharper in the Focus as far as I could tell and overall it just felt a little more agile.
The Focus' interior was not as good looking as the Golf's though, and the seats weren't as good either. One thing I especially didn't like was the remote control for the radio which wasn't integrated in the steering wheel, but was a large extra stick below the indicator. That looked horrible.
Some options that I asked for weren't available in the Focus such as cruise control and a couple others.
The gearbox was good, not as short shifts as the Golf, but still good. You felt a distinct click when the lever was in position.
After test driving I thought about it until the next day and tried to summarize the Focus and all I thought about was that awesome engine and engine sound where when I recalled the Golf, I remembered the entire package - seats, engine, gauges, interior, the sublime DSG box etc.
No doubt I would have been happy buying any of the two, but I think the Golf is the car I will be the most happy with most of the time. Plus I was really sold on that DSG after trying it.