Ok, so I think an ADC with at least 10MHz sample rate would do (100MHz would, obviously, be better), assuming you're just looking to discriminate between 250nS and 1000nS pulses. Texas Instruments(among others) has lots and lots of ADCs that would meet your requirements. You need to consider what other features you need. I think that 8-bits would be fine if you're only looking to determine which one of two pulses you have. If you need a more accurate representation of the pulse, then get more bits.
Barry