Markham,
I think its great to be able to go between the different platforms. It gives you better insight to the capabilities and helps you think about some things in a different manner. I do see recently more LUW positions comin available, but still plenty of Z/OS jobs out there as well. I have supported DB2 on all but AIX platform thus far and I have to admit that I am biased for the mainframe. I think the mainframe positions, can, sometimes, pay a bit better, but it mostly depends on you and how you sell yourself. I have been working with DB2 for about 14 years now and really do enjoy my work. Good luck with the new job offer.