Your local potential opponents are a really important factor.
In 15mm we are blessed with amazing figures:
Xyston (UK) - all killer no filler, some closer to 18mm, all a joy to paint, first stop for ancients
Forged in Battle // War and Empire (UK) - big packs, loads of new dark ages armies
Legio Heroica (Italy) - again, joy to paint, medievals
Museum (UK) - new Z range for ancients plus massive legacy range (with less variation)
Khurasan (US) - eclectic and generally great
Baueda (EU) - figs and camps
Then the older massive ranges, lie Essex, Donnington, Magister militum (Chariot), and plenty of others, including plastics from PSC (same as Lurkio), new PoD plastics, loads more.
For rules, for 15mm, I think the research bears out that ADLG, MEG and DBMM are most popular globally for ancient-medieval 15mm. But there are many more.
And (apologies for posting this again), until there is a sensible ancients forum that’s user-friendly, the conversations about rules and figures are much more interesting on Facebook now - check out e.g. Museum, Grendell, Mortem et Gloriam, L’Art de la Guerre etc etc.
And for 10mm vs 15mm there’s also Madaxeman’s blogs and podcasts - big Pendraken fan there!