SAN - once you get a feel for it - you will quickly learn there is more to it than disk management. And will become 100% of your work day. From Brocade to VTL - from App solutions to TPC. EMC to IBM.
Within your second year of TSM, you are just learning the "ins and outs", and you will also find, in some cases TSM takes out 100% of your day, weekend, holidays as well. You will find TSM has alot to offer as its maturing as well in version 6.
As both technologies are in high demand, and both are in a substantial rate of change as well - to include Cloud technology - I guess it depends how long you've been in "the business", Depending on your tenure you can quickly come to a cross roads.
Knowledge base is good; Hands on experience is better.
Shoot - after 20 yrs in the business - I too to this day - continue to study other skill sets; i.e. CISSP, Program Management(six sigma), Cloud technology; being SME already, allows one to migrate into new environments easily and provide the technicial enhancements. We've all started our careers in one form or another as a system admin but....
Being a CritSit TSM SME as I am, Disaster Recovery or Business Continuity Planning is the way to go, Here SAN/TSM/System Admin skills are bound to a solid foundation.
I just became certified in Butterfly software - as Migration Deployment Professional. Here Butterfly transitions between BUR software to include TSM upgrades. Learning this, adds to my tenure. Provides additional offerings etc...to "the business"
Up is Up - no matter which way you choose to go - and Ive offered a couple of them right here.
Good Luck in your career goals....