Hi Ron,
Let me preface this statement by saying that I am a Canon guy. However, I have been doing some Nikon flash research for a friend of mine who also is contemplating upgrading her on-camera flash to a hotshoe model.
The SB400 is definitely an upgrade to the on-camera flash. It seems to me that it is quite similar to the Canon 270EX. Here are a couple of pros and cons that I see in the SB400 (along with the 270EX). Nikonians, please correct me if I am wrong.
First, the SB400 is light weight. I am able to carry my little 270EX everywhere, either in the pocket of my photo vest or on the camera. It is so light weight that I don't even realize that the flash is attached to the camera. Additionally, being light in weight and having a low profile, it balances quite well when attached to a camera that is on a strap. The larger and higher profile hotshoe flashes often tend to unbalance the camera a bit and cause it to tip when carrying it on a neck strap.
Since I always carry this little flash with me, I always have access to fill flash outside and a limited indoor capability.
I consider that both the SB400 and the 270EX are somewhat limited in use, especially indoors. I virtually always bounce my flash because I hate the look of direct flash. Both the SB400 and the 270EX are capable of tilting for bounce flash but, cannot rotate. This lack of rotation capability limits the effectiveness of either flash when bouncing with the camera in the portrait or vertical position. IMO, this is the primary fault with the small flashes of this type.
Secondly, both the SB400 and 270EX are rather low powered and often do not have enough power to light a reasonably large area.
If a photographer chooses to select either of these small flashes to use as a primary flash, I strongly recommend using a Joe Demb Photojournalist Flip-it diffuser reflector (
www.dembflashproducts.com ). This Flip-it helps when using the flash in the portrait position and it also helps in general bouncing because you can adjust the reflector angle to direct a greater position of the light forward.
"If wishes were horses then beggars could ride"; but I really wish that my 270EX (and the SB400 if I were shooting Nikon) could be used wirelessly. It would be great to have a second mini flash available in my pocket when I needed it.
NOTE: Canon has recently introduced a 270EX ii model which can be used wirelessly along with a 550EX or a 580EX [series] master. This is a significant increase in the capability of the unit - now, if they only come out with a 270EX iii that can rotate as well as tilt! )
As far as the Sigma flash goes. I have no experience with this unit, but it appears more fully capable (along with being larger and heavier) than the SB400.
BTW: Both the Canon and the Nikon 50mm f/1.8 lenses are excellent choices for a low light, general purpose prime lens at a very reasonable price.