If your using a spotting scope and excess bino weight bothers you then by all means get the 32mm objective ..If not then go the 8.5x42 route. The binoculars you are mentioning are light with not a big difference between 32mm and 42mmd you will have a big difference in the twilight factor between them either at 8 or 10 power. the twilight factor at 8 power for the 32mm bino would be 16(32mm) vs 24.41(42mm) and for the ten power it would be 10.24(32mm) vs 17.64(42mm) so there is a big difference in the 8 x vs 10 x and you' ll be able to hold the binoculars steadier with the lower power.