You can start with the second, but the first book is better at grabbing the attention of a new reader with wild ideas (broadband audio hive-minds, variable speed-of-light). If you make it through the first chapter you won't be able to put it down.

The second book is just as good, but doesn't try as hard to get you addicted early on. The assumption is that you already know how good Vinge's work is.

I recommend starting with Fire Upon the Deep.