For single long exposure shots the P45+ Hands down. I own this and use it on a H2 which shutter works in 1/3 stops up to 18H 12M… Also the P45+ can be powered by a canon dual charger to power the back for long periods of time..
An example using a P45+ with a 1 hour 26 minute exposure.
1 hour 26 minutes, Peaks. by
RickrPhoto, on Flickr
exposure stacking also is a very useful method in my experience, and both are useful in certain situations. I would suggest using the stacking method over a single exposure if it is within a week before or after a full moon, and the moonlight would wash out the sky of the image in a single exposure. Ironically a week before a full moon is one of the best time to do star trail landscapes..
This example was a stacked exposure taken with my 5d mark II over 3 hours. Also you gain the ability to edit further with the stack, I.E. a car drives through your frame which you may or may not want the light painting effect.. just grab a shot with or without it from the series and tada… easy fix.
Monument Valley Star Trail by
RickrPhoto, on Flickr
If I was looking to invest in a new system specifically for long exposure night photography, I would personally suggest a D800E as you can stack the images pretty well, and it seems overall easier. The P45+ needing to do a dark frame, not to mention the difficulty of guesstimating what exposure to use, then having to wait twice as long to see if you were right could possibly leave you without any shots.. where as stacking lets you use the camera twice as long in the same spot to get longer trails..