It depends what you want to take pictures of. If you plan on taking pictures of people, consider a 50mm or even a 75mm. If you tend more toward landscapes or street, you could go wider than 35mm, but 28mm is pretty close and for anything wider you'll need a different body or to use an external viewfinder.
I personally really like the combo of 35mm and 50mm. I think that each is very versatile, but they are definitely different enough to make things interesting. Plus, there are lots of top-notch 50mm lenses that are relatively inexpensive (Hexanon, Planar, pre-ASPH Summicron).