I recently wrote an article going into detail on the features, pricing, discounts, and my experience with each.
Which one you use depends on your individual needs. If your main concern is the cost, Bitcoin.tax is much cheaper. If you have a lot of alt-coins or use an advanced portfolio tracker, you probably want to go with Cointracking.info for its extra features.
You get a 10% discount if you purchase a Cointracking.info plan using the affiliate link above. You get an additional 5% discount if you pay for your plan using Bitcoin.
I could not find any discounts on Bitcoin.tax.