Wacha is starting Thursday vs. the Royals and Wheeler is expected to get called up in the very near future.
As far as fantasy I believe that Wacha's ceiling is much higher due to the fact that he is on a contender and I have not seen anything mentioned about a potential innings cap like I have seen with Wheeler.
Pretty sure Wacha is going to be in the rotation for a while because Jaime Garcia and now Jake Westbrook are out indefinitely. Last I saw Westbrook was going to see Dr. Andrews for his elbow which is the fantasy kiss of death.
Wacha is a Matt Harvey light. Command of 4 pitches with a mid 90s heater. He's a K guy despite his AAA numbers. shite he was 17k per 9 last year. He doesn't walk anyone or give up many hits. He has flat out dominated minor leaguers. Wheeler has been up and down and doesn't deserve the hype IMO
Wacha's high K numbers were a result of throwing 2 innings at a time, which allowed for increased velocity. I think he'll be around 7 K/9 in the majors, if throwing 6 innings at a time (Which is still decent). He has a great changeup, good fastball command (which plays up due to the CU) and an improved breaking ball. Guy was a stud at A&M last year and shouldn't have lasted to pick 19 in the draft.
Wheeler has the higher ceiling, but also has a little more risk. His fastball has great movement and he throws a bunch of GBs. His curve is a plus pitch and his CU has improved. His shoulder injury is a concern, because shoulders can turn into career enders, but it appears it was nothing serious. His throwing motion had improved since he was with SF, but it still shows inverted W.
If you're risk-averse, competing now, and chasing wins, Wacha is probably your best bet. If you're swinging for the fence, Wheeler has true #1 potential.