I Think this is really necessary for you to study a complete datasheet of one ADC, so that you can get familiar with different important parameters of an ADC.
Then the really important point, is to what device you want to connect this ADC ( as Fox says ) for example, suppose that you want to connect this ADC to a DSP from TI, then it is really better to choose the ADC from this company since the ADC can be connected directly to the DSP without any need to any additional glue logic. Or when connecting to Analog's DSPs, it is better to choose the ADC from that company. This way the design time can be reduced and you can get better results.
Any how, you should indicate the needed precision ( as FOX says ) because this is a really important factor.
Also you should indicate if you need a same precision during all of the 0 to 50MHz frequency band, or when the frequency increases then we can say that we need a lower precision?