There are some tip styles made in 900M that aren't available in T18. I use 900M in this case, and it works fine.
The T18 are slightly thicker in the base where it slips over the ceramic heater, which should provide better thermal transfer and less lag in response to the temp probe. That's the only difference.
What little use I have made of fake tips, they seem to be made perfectly fine. I even cut one in half, and this particular one would probably last 30 years, the iron was so thick. But the tips I use 99% of the time, I have never seen in fakes, where you get hmm, basically, straight conical, standard large bevels, and chisels, and a knife, and that's about it. Well, that's actually a pretty good variety for most people, lol.
In my experience so far, the cost of genuine tips is not a realistic concern. It will be multiple $20.00 rolls of solder before I wear out a Hakko tip. But the fakes I have seen would probably last just as long.