Flash for D3100 (built-in flash creates shadow with most lenses)

I wouldn't get the SB400 as it is very underpowered and you may end up spending more later for better bounce capabilities which will get around any lens/hood issues. The larger models have better remote CLS capabilities as well.

The SB600 is no longer made.  I started with one, but it didn't have the flash-fill reach for me outdoors either at all beyond maybe 6 feet as I recall. You might try the SB700, or even SB910 if you need more power for outdoor fill. There are also cheaper 3rd party units too.

Mack