Howdy Troy, welcome. Man! both are awesome choices. I have a Honda with the Grubee GB and can't say enough good about the engine. Really, smooth and lots of power. I have no experience with the Mitsubishi.
I really want that GB!
(from their web page)
"Our 18.75 to 1 gear box has a Life Time Warranty on the Gears. If any gear fails EVER we will replace it at no charge. We have many years of our gear box in use and know they are by far the BEST you can buy. Our gear box is by far the very best quality and very, very Quiet. We have current customers who have more than 60,000 + miles on our gear boxes and they are still running strong. Our gear box will NEVER need replaced and it's made to last a Life Time and them some. Made in the USA. We have been building our highest quality gear and chain drive kits since 1984."
Well I posted a lot and was of lil or no help, lol
Anyway, welcome and keep us posted on what you decide and please post a review.