I ride a 750 everyday to work in Melbourne and play on the weekends in the hills. I chose the 750 over the 900 (at the time - was before the 800/1000!) because everyone I spoke to said the 750 was bulletproof for commuting and the wet clutch was easier to deal with.
Two years on I can tell you that I am still rapt with the 750. Yes, bigger bikes clean me up in the straights and at the lights but for commuting and hitting the twisties the 750 is fine. Having said that, the elec injected 800 has a few more ponies as stock and the 1000 a few more still.
My point is, unless you are an absolute gun rider you would probably be happy with the 800.
Personally, I would get the 1000 just for the extra power on tap. I am 6' and 73kgs and the bike even the 750 hauls me and my wife around admirably so thats not really a concern.
I would love the S4 and have seen them 2nd hand for $14k but they are a 4 valve engine and that basically doubles all your servicing. My wife nearly died when I got my last major service bill so....something to consider...
Go with the S4, you'll love the extra power. Also, when hauling two people it's nice to have more power. A few of the guys in my group have 800's and 750's and said they wished their bikes had more power when hauling another person.
Seven50 is right though. The service bills on the S4 are killer. You'll want to learn how to do a lot of the maintenance on your own. But you can figure the 6000 and 12000 mile checkups to cost somewhere between 500 - 1000 US.