Play the original first. The voice acting is much better and the pacing and and combat and platforming is still tight. Without the context of the original, Black Mesa won't be as impressive. It's a great effort, but it's pretty clear it's not a professional game.
Speedrunning is not really a viable way to judge the game's completion time. Howlongtobeat.com rates the original Half-Life at about 10-12 hours, which seems to gel more with my memory. My own Steam record says I logged about 10 hours on it.
I would keep playing through HL1 and then BM personally. There's some changes like to grunt AI (BM is based on Combine AI) that make the original still worth playing.
There's also some features that are broken in the Steam release of HL1 vs the original Sierra release if I remember correctly but that's another bag of worms.
Black Mesa - definitely. Preserves everything in HL1 and adds a lot more. Xen is like 100% different though, Black Mesa's one is waaaaay better but if you want to experience HL1 version...