I'd go for the lens.....I already have the 360 and 540 flash units, and have used them considerably in the past, but seldom with the K5, since they are seldom needed with the great high ISO.
I do use them sometimes for macro work to black out the background, but otherwise they sit on the shelf 99% of the time...which is more than the 35mm would sit there unused....and at 2.4 you would seldom need a flash with it...if ever?
I'm not much of a flash fan anyhow........excepting for studio shooting, I try to avoid flash. But....that's just me, others may have a different view?
Happy Birthday !
Regards!