Which do you prefer to write? Fiction or memoirs?
Fiction writing has some great advantages over memoirs. When you're recounting something that actually happened, a lot of times the story would be more interesting if it had happened just a little differently. With a memoir? Tough luck! With fiction? No problem!!! :D
Since you don't have to be true to the facts, fiction gives you more liberty to be true to the story.
Still, I love writing memoirs. I've written far more memoirs than I've written fiction. I think memoir-writing is good exercise for writing fiction since it requires the same storytelling skills.
When you're writing a story that actually happened, it's not as though it has been recorded in your brain in some canonical form and you just need to download a copy from your brain onto the paper. You need to craft the story: decide what to include, what to skip, and how to say it. Even something as basic as deciding where an episode begins and where it ends is part of the difference between a memory and a narrative.
(Ninja writer C.V. Rick is an example I've stumbled upon recently of an excellent memoirist who has been turning memories into engaging stories and posting them.)
Personally I haven't written much fiction at all in the past year. It's not that I don't want to write fiction -- it's just that I don't want to sit down with the intention of writing fiction. I'd rather wait until I feel like I have a particular story to tell. That's how I've come up with all of my best work.
At the same time, I want to stay in practice so that when inspiration strikes, I'm ready. That's where memoirs come in: they're storytelling without the story-inventing part.