Thanks for all your helpful comments. I think you more than convinced me to sign up with Marriott's reward program now. I was hesitant because I live in Asia and most of the places I travel to in the Region don't have a Marriott where I need it. But if 25 nights are enough for silver, I think it's worth it anyway, esp. if I can convert the points to Airmiles later if I really don't know what to do with them.
The credit card sounds tempting, but not being a US resident it doesn't make much sense for me.
What exactly is the Concierge level? Is this Marriott's club floor?