i am reluctant to suggest a specific one.
becos tanix is not great with clarity and transparency with firmwares vis-a-vis their model variations.
try google and read up, incl this:
https://www.tanix-box.com/downloads/
tx9 is rather old AML s912 chipset.
tx3 is rather weak AML s905W chipset.

if prime objective is astrogo/playtv, i actually suggest u get tx3max.
to be sure the model u buy will work, ask the seller(s) the specific impt questions before u buy:

.. 2+16gb?
.. android 7.1 or 8.1?
.. firmware version?
.. Alice UI or ?
.. option to close root?
.. can run astrogo, playtv?
.. BT?
.. price?

yes, i am using tx3max firmware on my tx3mini.

but, please read up and not get caught in a bind where u feel like a brick...

incl what i experienced as per below and related subsequent comments.

all the best.

READ post #458
https://forum.lowyat.net/topic/4722095/+440