PicUp3D is a PIC simulation software that was created before SPIS by Julien Forest.
You can find out more on PicUp on the dedicated web site
http://dev.spis.org/projects/spine/home/picup.
There are a lot of differences between the two software.
SPIS uses unstructured mesh while PicUp3D uses cartesian structured mesh. The models differ as well.
SPIS is more configurable and offers more possibilities for S/C charging modelling
Regards