The headtube isn't threaded or threadless, only the fork is. You can use 1" forks in a 1 1/8" headtube using reducers, but this is far more difficult than using a 1 1/8" stem on a 1" fork. A 1" headtube gives you the most options for stems, but 1 1/8" is quickly taking over the fork market, so if you plan to be shopping for aftermarket carbon forks, 1 1/8" might be better.