To me, Splash is easier on the body by far. It's practically a relaxing ride for like 90% of its duration and very whimsical and sweet. Yeah, that last drop is frickin huge but it's over in what? like less than 5 seconds? Yeah, that's enough time for your stomach to drop LOL but it's still such a brief bit of coaster fury.
While Big Thunder Mountain Railroad, on the other hand, is basically full throttle the whole entire way and will jerk you this way and that, side to side, bounce you up and down and rock you through the skies! Talk about shake, rattle and roll! It's in no way a mild experience.
Like, I'd recommend for elderly people to go on Splash Mountain way before I'd ever endorse Big Thunder Mountain Railroad for them! If you have any sort of hip, neck or back issues, proceed with extreme caution before boarding Big Thunder Mountain Railroad! It's in many ways one of the most physically disruptive rides in all of Disney World even if it's lack of MPH puts it behind, say Test Track or it lacks the bells and whistles of say, Rock and Roller Coaster (which very securely keeps your body in place despite it's tumult). Seven Dwarf Mine Train is also a ride that is far more smooth and BTMR. Space Mountain bounces you alot too but doesn't sway you side to side or feel as rickety as BTMR.
It's sort of like if Dinosaur at Animal Kingdom was a roller coaster - a ride guaranteed you jostle you, so just be sure you're into that.