They are pretty close dimensions as below, however are very different foils.
977 is more of a race foil, high top end but relatively higher stall speed.
877 has a much lower stall speed so will get going much earlier and keep gliding for longer, but doesn’t have the same top speed.
877 is 877cm^2 / 910mm span / AR 9.4
977 is 977cm^2 / 950mm span / AR 9.2.