Size, capacity and weight! Both models offer the highest level of protection from waterborne pathogens (99.99% of virus, 99.9999% of bacteria, and 99.9% of protozoan cysts, including Rotavirus, Hepatitis A, Norovirus, Giardiasis, Cryptosporidium, E. Coli, Cholera, Salmonella, Dysentery). They also filter many heavy metals, chemicals, and microplastics. GeoPress filters and purifies 24 oz of water, ideal for individuals and groups looking for a high capacity. UltraPress purifies 16.9 oz and is great for individuals with limited space or when traveling with small pack side pockets.