I'd take a look at the places that you travel to mainly and see what hotels are in the area that you might like and are convenient, and then focus on that brand.

I branch out after making sure that I have my Starwood Platinum status renewed for next year. That doesn't mean that I don't stay anywhere else until I have my nights/stays for Starwood, but I at least make sure that I have those nights/stays coming and then branch out to my backup (Hilton).

It's definitely good to have a backup. Starwood hotels just aren't as numerous as other chains are, especially in the lower end hotels and in smaller cities/towns. That having been said, I don't think I would branch out if you are just barely making SPG platinum.