David mentioned range,that's what I think is important on an air rifle,you'll only need to light up a target clearly,as in can identify,at 40/50 yards,eye shine probably 80/100 to let you get near then shoot,so I think that is worth considering as well as the other details on the torch itself...
Mines a Chinese cheapo but works well enough with a filter on front at the range I choose,I also wrap card round the end to keep the spot really tight,probably only a few yards across at 40 yards,the stock mounted pressure switch is the thing to look out for really as you can adjust the light a bit