I would recommend the OxyPowder cleanse. Because I also suffer from constipation, and while I was taking Colonix last year, I would sometimes have a hard time having a bowel movement. As for side effects of Colonix, I didn't have many. I only got bloated and constipated. I've also heard of people getting wicked headaches. I just completed the OxyPowder cleanse a few weeks ago, and it helped me with my constipation.
Colonix is a good product though. I don't think you will go wrong with either one. I just think that my body reacted better to the OxyPowder.
But if you don't know much about OxyPowder, then you need to know that it's a completely different colon cleansing product. Colonix is an Herbal Colon Cleanser, because it has 40 herbs that work to detoxify the colon. OxyPowder is an Oxygen Colon Cleanser, which doesn't use herbs to 'bulk up' & 'push out', it liquefy's the stool and it's much easier to pass thru a bowel movement.
But you already are on the right track. Both cleanses will work, just try to figure out whats best for you and your body.