Unfortunately you can't put his injury aside; he is a major injury risk because of his elbow. Yes, Stroman may have something come up, but that is a given for any player. Tanaka's injury isn't just a possibility, it's reality. What happens if he totally blows out his elbow this year. Then it's TJ surgery, which is a minimum 12 month recovery. So let's say he pitches 1/2 the season, blows out his elbow, and doesn't come back until half way through 2016, which would likely be at less than 100%. That's 1.5 seasons lost in 3 years, and not a very good return on a very large investment. Maybe he survives the whole year without injuring his elbow, but at some point it's going to blow.
I think Tanaka has a higher ceiling than Stroman, but for me, the injury risk is too high for him, and if I was picking someone from the above list to pitch in my rotation for the next 5 years, I'd pick Stroman because he is the safer bet, even if he isn't the better pitcher.