I would recommmend doing both parks. My friend just did both parks in one day back in July, in the peak of summer, and it is very possible. Of course, you will miss
some things, you won't see every single thing in either park, but if you plan wisely, you can squeeze a lot in at both parks. I think you should see California Adventure while it is still in its sort of current 'format.' It is going to dramatically change in the next few years. Make lists, have a good plan of action of exactly what you really want to do, and it is very manageable!