Generally speaking, Force eTap AXS uses less expensive materials and manufacturing processes compared to RED eTap AXS.
Some chainring options, like the 50/37 2x chainring combo, and the 50T 1x Aero chainring, only come with RED eTap AXS cranksets (but those chainrings can be installed on Force cranksets if purchased separately).
Internal technologies are the same for both groups, so performance on the bike will be indistinguishable, but Force eTap AXS will weigh about 300g more than RED.
MSRP will be about $1000 less for a Force eTap AXS group set, depending on the configuration selected.