Hi Susan,
I have a Digisix and it's a neat little meter even though changing ISO and operational modes is a touch fiddly. I can't recall the specs precisely, but I've a feeling that it's a couple of stops more sensitive in reflected mode than in incident mode. However it doesn't hold any significant advantage over the meter in my RTS II, and I only tend to use it when I want an incident reading (or with other cameras -- it's a great partner to my Leica M6).
For low-light work (dawn/dusk and indoor artifical light) I expect you will be quite happy with the Digisix, but I would hesitate to recommend it for full night photography. For that I would suggest the Luna Pro S or better yet the Luna Pro SBC (or the Pro F which is like the SBC but adds flash metering, if that's useful to you, and incidentally there is now a Digiflash which is the equivalent of a Digisix but with flash metering).
Of course in night photography metering is much different than in normal or low light, in that the subject is often distant and tends to consist of pinpoints or small areas of light. Incident metering is usually irrelevant and "reflected" readings (emitted, really) won't necessarily be much good either. There are many rules of thumb that can be applied to useful effect, and there is a thing called The Black Cat Extended Range Exposure Guide which summarises many of them on the back of a simple rotary calculator that can be used like the scale of a traditional meter. I haven't used one myself, but it seems like a good idea and is widely recommended on photo.net. Have a look here for further details:
HTH
-= mike =-
