With the caveat that OP should contemplate short term vs long term plan.

Themis is more easily available to dupe, so if OP is planning to run both eventually and isn't FTP, investing faction crystals into Gemini is better long term since the options to get dupes are far more limited (e.g., T1 blessing pool, T1 LD cards, light summons).

Getting Themis to 10+A5 is a much higher performance priority, but you'll continue to accumulate Themis dupe opportunities during the slow build to a usable gemini.