To that I say, weeeelll, it's a little of both.
Regular readers who visit here know I've been revising Long Road for almost two years now, but I don't expect that my next project will take nearly as long.
Since Long Road is my firstborn (in a literary sense, anyway), I've spent a lot of the revision time honing my skills, learning how to create suspense, how to weave in subplots and how to discern which scenes are necessary and which scenes are frivilous.
This is stuff I will know once I start writing Whisper, which at this point I'm merely researching, outlining and establishing characters. (BTW: I have an official name for Whisper's MC. Anyone care to guess? Think child of hippie-chick, but not as far out as Dweezil and Moon Zappa.)
I've purposely prevented myself from writing sections for my latest project for two reasons.
Number one: I don't want to drain the creative juices I need to see Long Road through to the end.
Number two: The longer I wait to write, the more vivid the scenes become. The more vivid the scenes become, the easier they burst forth when I do sit down to write. (Kind of like when the bell rings and the kids come rushing out of the school.)
The only issue I have with irons in the fire -- and this is pretty teensy-weensy -- is I have a tendency to start reading several different manuscripts, books or stories at once.
I'm a slow reader (hey, I like to soak up the words like meat in a marinade), so when people ask me if I liked something that I was reading a week ago, my face turns a couple shades of red as I admit I'm only halfway done.
Fortunately, those are the only irons melting in my fire.