I have the Pocophone F1 and I would say it is a decent phone.
I'm gonna say between Poco F1, Mi 8 and Mi 9t, there's not much difference. But i'll try to look at the best feature in them.
Mi 8? It has the Super AMOLED, corning glass 5, HDR10 screen, best screen. Great camera (+telephoto, OIS), sufficient enough. SD845 Adreno 630 on par with Poco F1.
Mi 9T? The latest device. AMOLED, corning glass 5 screen. Camera is supposed to be the best. 48MP (+telephoto, ultrawide), under display fingerprint (if you like that, idk for me). Headphone jack. USB OTG is there to compensate for the less expandable storage. 18w 4000 mAh battery.
Poco F1? SD845. Expandable storage, headphone jack, stereo speakers, USB OTG, 18w 4000mAh (QC 3.0) battery. The lowest cost among these three, or probably the same cost as Mi 9T (depends on your country, idk)
Of course there's pros and cons between them. And I saw a lot of pros in Mi 9T that actually makes me eager to get it (if only I hadn't buy Poco) Anyway choose one that fits you.