It would (in my mind anyway) depend on what market you're looking to work in.
If you're already working in a MS environment (and like the OS / SysAdmin end of the stick) then I'd say, go for the Linux .... It's getting big enough that organizations are starting to migrate (or at least do some pilots). Having skill in both would make you a good bet for a "migrate and maintain" shop.
If you decided to move into hardware, then go with the CCNA (as a starter). CCNA is a pretty basic cert (better than it was, but still an entry-level flavor of certification). That, coupled with some MS background, can move you into VoIP or provider networking ... where some of the job is hardware, some of the job is OS / Software / Web / Content.
As a possible peripheral specializaition, consider wireless & wireless domain stuff. There are a number of hardware folks getting ready to crank out 802.11 + GSM "cell phones" that will be "PBX-to-the-Parkbench" enabled (work anywhere, dual-mode, VoIP/Cellular hybrids). Having some OS background, LAN/WAN background and wireless would be a "Good Thing" to have on the resume.
Security is also hot right now, but by the time you have enough experience under your belt to be good at it, there will likely be a minor glut on the market (there are a lot of folks working on security certs (like CCIE Security), most have considerable experience already).
What do YOU like to do? What are your interests? If you're just chasing the cash, don't bother ... It's not enough to know it; you have to like it and live it to be really good at it.
.02
Scott