Well, it will take long to tell the whole story behind. But i’ll just say that Spark is alive and some work is done all the time. Small and big changes. Currently there is only one community member doing the developing. Jive is not working with Spark anymore. You can always test with latest version by compiling the source or checking this document http://www.igniterealtime.org/community/docs/DOC-1822 for the installers.
Eventually i think (i hope) there will be a new release, but i can’t give you any date.
I think they are all similar in the stability. I mean, that Spark always had some issues with that. Well, it’s not crashing, but often it would lock up or freeze a bit. But that also depends on a machine and resources. I’m often experiencing such issues at work with the older computer with 2.6.0 (and previous versions). But it works fine at home on a newer and more powerful box with 2.5.8, 2.6.0 and latest SVN version. Windows XP on both. Could be some software conflict though.