Probably the YN since it has more power. I have the SB600 and it can work well as a fill flash but don't expect to blow out out the sun or anything like that!
YN is cheaper, I hope you are talking shoot through umbrella, not bounce. Neither flash will do much outside if bounced through an umbrella. And make sure to get sandbags, umbrellas have a tendency of blowing away.
Can I ask how you plan on triggering it off camera? Here's my recommendations:
- If you plan on using TTL at all (using a commander mode, SU-800, or - my choice - Phottix Odin's), you would want to use the SB-600 as the YN doesn't support TTL. Keep in mind though that using optical triggers (commander mode/SU-800) will be a pain outdoors!
- If you plan on going manual, I would recommend the YN for the extra power (almost 2x). You can then get yourself some YN-603's with it (~$30) to trigger it wirelessly (or go for the YN-560 III with the wireless receiver built-in!).