Exterior styling, C-Class feel more luxury. The paint quality of M-Benz is better than 3-series, too. (M-Benz use nano ceramic clear coat which will stay shinny longer)
Interior styling: If you have Nav screan, 3 series feels more luxury. However, without the iDrive/Nav screan, 3-series looks ugly and cheap.
Engine: 3 series win hand down.
Transmission: MB has an slight edge. (7 speed vs. 6 speed, but BMW is made by ZF)
If you can't get the deposit back, stay with 3-series. otherwise, i like new C better.