First of all I think it is stupid to compare two microcontroller types in general. In any field you may find something that fits your particular needs. When you start building a new project you define needs and then you pick microcontroller that fits. If you choose microcontroller first and then think of properties, then probably you should rethink the strategy. This applies more to production level. In hobby market things always works different. You can always find many arguing why one microcontroller is better than another. In that case you might find interesting comparison prepared by majenko.
In his table he put three popular microcontrollers side by side and compared common features. The microcontrollers are: Atmega328P found in Arduino, PIC18F25K80 and MSP430G2533. In peripheral war PIC is obviously a winner. It offers better ADC, more PWM channels and more 16 bit timers. The MSP430 looked worst in that way. But don’t get caught by this comparison, because it doesn’t include tools. PIC doesn’t shine when you need a programmer. PICKIT 2/3 is more expensive then AVR ISP. In other hand MSP430 offers low power capability. As always you should use your own brain when choosing, but it is interesting to read other opinion and thoughts on microcontroller choice.