We have a Sunpower system (Sunpower panels, rebadged Sunny Boy (SMA) inverter). We used Baker Electric Solar (Escondido, but they install all over the county - we're in Sorrento Valley). Very satisfied - super clean install compared to many I have seen, and have referred several others (if you want to talk to them, PM me and I'll put in a referral, if you end up going with them I'll take you to dinner with the referral bonus
The parent company has been around a long time, the solar division is newer but seems fully matured now. The wiring job we got seemed to benefit from their home electric expertise (conduit in wall rather than all over the roof and side of house, stuff like that). They were one of the first big Sunpower dealers in town - now from the ads it sounds like Sullivan is also selling Sunpower (they were doing Sharp and BP when we were shopping).
We paid on the order of $5 a watt but that was well over 4 years ago, it was competitive at the time and prices have come way down. Also our system is pretty small (3.44kW DC rating) so that wasn't in our favor on cost per watt. Our next door neighbors just got a largesystem from Baker and paid much less per watt, though I don't know the exact figure. You should get several estimates, it will become pretty clear where prices are these days - the California rebate is either over or tiny now, so the main incentive is the federal credit, and there's a lot of competition among the vendors now.
We have a single inverter, microinverters were just getting going back then, but now I'd probably look closely at microinverters if we went to expand our system. Except...I'm also intrigued by the new Sunny Island inverter which while grid tied can actually still be used in a mode where it supplies power when the grid is down (not to the entire home, but to an outlet).
I have not heard of Solarworld panels.