Average Aragonite:Calcite ratio in biogenic CaCO3 in the oceans
After some more research, the conclusion is that aragonite is somewhere between 10 and 50% of total CaCO3 production:

Aragonite, calcite-Mg and calcite are the 3 main species of marine CaCO3. Aragonite is unlikely to be a major component of the total CaCO3 flux (Berelson, 2007), though the exact contribution is uncertain and falls in the range of 10-50% of global CaCO3 production (Gangstø, 2008). Minimum values of 12% (Berner and Honjo, 1981) and 13% (Fabry and Deuser, 1992) have been reported based on sediment traps. Calcite-Mg contributes ~8% (Fabry and Deuser, 1992) to 24% (Morse, 2006) to CaCO3 production. The sediment trap-based contribution of aragonite of ~12% is thought to be lower than the actual contribution of aragonite to CaCO3 production because aragonite is about 1.5 times more soluble than calcite and therefore more depleted than calcite in the sediment trap records.

An educated guess of me would therefore be ~20% aragonite, ~10% calcite-Mg and ~70% calcite globally on average.