Well, assuming that you have the correct mid pipe (X or H) to fit the midlength headers I'd go with them. According to Bassani, they are the optimal length for hp and torque. I've only seen them dynoed in one mag and they outgained MAC longtubes by a fair margin.
If you don't have a bigger throttle body, the accufab plenum isn't worth much. A switch to midlengths and a different mid-pipe (required) would be a pretty penny, into the 700s I'd wager. I've never seen the Accufab dyno over 12hp, and that's with a new throttlebody.
Basically they are both solid mods, how much do you want to spend to go fast? I'd take the bigger hp gain if I had the cash. A plenum w/o throttle body is a small gain.