Hi CaliforniaLover,
I can't really give you any specific advice - especially about your particular industry. But there are a couple thoughts that come to mind. Hopefully someone else out there can give more specific advice to your particular situation.
The biggest piece of advice I can say is to do what you truly want to do, regardless of what may seem the best thing to do. I know that sounds corny, but my experiences are that you'll never regret it. Especially when it comes to moving across the world. If your life is in a place where it's feasible right now for you to do it - then go to California if that's what you really want to try. It may not be as easy to do in future years.
For me, I moved for school. I had to choose between a very prestigious university in the US, and the school I'm going to now. Where I am now (in Australia) has given me the opportunity to live and do fieldwork in Papua New Guinea. This was an experience I couldn't turn down. And truthfully, yes, sometimes I think "what if I had gone to that other university? I'd graduate with a degree from there, and getting a job would be so much easier. Especially if I decide to go back to North America." But overall, I have yet to truly regret my decision, because the things that I have done - particularly with respect to living in PNG - have been truly life-changing. And wonderful.
Now, on the particulars of your situation, and job prospects, etc. Isn't California where all of that computer stuff goes on? Sorry for my ignorance in this, because I really don't know much, but I thought that if you were in the computer industry, California would be the place to go. Of course, I really have no idea.
On another note, do you have any job prospects in the US? As far as I understand, it's quite hard to actually move there without a job lined up. And even then it can sometimes be difficult. But I've never done it, and I'm sure that there are other people on here that can advise you better on that (although you may already know!).
Also, have you checked out this post on the American Dream? I realize that this is likely not the reason that you're considering the US, but special k in particular says some encouraging things about the attitude towards Europeans living in the States.
Also, I think that it's good that you've been to California - you know what to expect. And sure, there are huge differences in some ways to living somewhere and visiting a place. But I think that this also depends on what type of visitor you were. If you just did all the touristy things, then this is probably very true. But if you mingled more with the people that live there, you might find it less true than you think.
Oh, a couple more thoughts. As far as I understand, it can be very difficult to find a place to live in some areas of California. I know I was once considering moving there, and here that commuting times can sometimes be hours! Obviously, this depends on what area you're going to. Also expensive - as far as North American standards anyways!
I have absolutely nothing to offer as far as London. I've never even been to Europe (with the recent exception of two hours in the Frankfurt airport - doesn't count at all!). So I'll leave that to someone else.
California is a beautiful place. (Been there twice myself.) And as you probably know, the culture along the west coast is much different to the rest of the US.
Hope something I said was useful! Good luck with your decision!