Despite all of the perfectly good arguments for staying with linux I think
that by installing NetBSD on one of your machines you will find that you have
a much more versatile environment, because as you stated you already have two
linux machines moving one over to NetBSD will give you one of each.  Also if
you are interested in using the GUI software, as you stated I didn't seen any
mention of NetBSD support regardless of architecture.  This means that by
installing NetBSD you will be able to work on any NetBSD specific issues which
might arise; from the comfort of a full featured workstation, as the dreamcast
lacks some rather important features such as secondary storage :-)

Tyrel