Robbie,
Go with the heavier engine. Slow Pokes come out tail heavy. I've seen builds where they needed 10-16 oz of weight in the nose. I was very carefull when I built mine and build the tail as light as I could, moved the receiver as far forward as I could, and put the battery under the fuel tank and I only needed 2 oz of weight to get it to balance. So a little heavier engine will help you out here. I used an OS 52 Surpass in mine which weighs 14.2 oz.

Ken