You know what the difference between 3/4 and 9/8 is, right? 9/8 actually represents a quicker pace than 3/4. But besides that 9/8 is a compound T.S., meaning it carries a background of 3 (sort of like a triplet). Da da da (repeat).... is clearly a background of three. Three clear pulses between beats. 3/4 would be like Da da Da da Da da, cause it's a simple T.S., meaning a background of two. Each beat has an upbeat and a downbeat. That's not how that song feels.

I'd say it's 9/8 because there is clearly 3 beats, each diviable by 3 (like a triplet). I can see where your saying it's 3/4, because that's basically the same thing although each Da da da is a seperate measure of music. That's ok too. It's just that Da da da is the basic pulse throughout the whole song, which is clearly compound (feel). Then there is the alteration of 3-4 beats before each change. I dunno, that's just how I see it.

Also, remember tempo has nothing to do with time signatures.