Well, both learn the same HM moves, so both can be good use for an HM slave
Talking competetitively, I'd say choose Empoleon, but heres some pros and cons of useing Both
Empoleon
Pros
-Steel typing gives it a lot of resistances
-Has a larger movepool than Samurott
-Better defenses
Cons
-Slow
-Less HP than samurott
-Being steel type gives him weakness against the popular earthquake
Samurott
Pros
-You can make him a mixed sweeper
-Has a high HP stat
-Has more type coverage than Empoleon
Cons
-His speed is still low, and his defenses cant make up for that
The decision ir yours tho, hope I helped