If you look at the statistics for both players it will tell you which player has had the biggest impact.
For any comparison to be meaningful it needs to be based on a level playing field so if you look at goals scored you will need to deduct the 5 penalties scored by Sinclar, simply because Bellamy doesn't take penalties for Cardiff.
So in open play Sinclar has scored 11 goals and Bellamy 8 but if you compare these stats against time on the field Sinclar has scored a goal in open every 253 minutes he has spent on the field, Bellamy has scored a goal in open play every 246 minutes he has spent on the field.
As for assists Sinclar has provided 1 assist every 2800 minutes he is on the pitch, Bellamy has provided an assist every 400 minutes he is on the pitch.
So purely on match statistics Bellamy has been more effective this season. Simples.