I think they're sort of hard to compare, as they're quite different. Splash is a log flume ride that floats along the water, has a few smaller drops, followed by one large one. BTMRR is a true roller coaster. A rather tame one for sure, but a coaster nonetheless. It's much noisier (if that bothers you), it's faster (except for the big drop at the end of Splash)...though it's not "fast" by coaster standards. BTMRR is fairly "jerky" as it goes up and down a few times fairly quickly and has turns taken at speed, where Splash doesn't. The turns on BTMRR are also banked in spots, where Splash isn't.
To me, all in all, BTMRR is more intense. The singular drop on Splash is definitely much larger, higher, faster for sure. But the rest of Splash is less intense than BTMRR. Not trying to scare you at all, OP. As I said before, on the scale of roller coasters, I see BTMRR as very mild (maybe a 2 or 3 on a 1-10 scale), but it's still a coaster. I suggest trying it, but it's really hard to compare it to a log flume.