Hi there and welcome to Blade Forums.
I'm not going to be able to really answer you question but wanted to welcome you just the same.
My experience with both steels is very limited. In fact I don't think I've ever used Buck's 420HC but have read nothing but very good stuff about it. Seems they really nailed the heat treatment on it. My only experience with 420HC is with a couple of Condor fixed blades and I was very pleased with it's performance.
I also don't believe I've ever used Benchmade's N680. Again, only from what I've read it seem's to be a decent steel offering very high corrosion resistance with very good edge holding. A good mix of qualities in what might be called a more affordable steel.
Again, sorry I couldn't be more helpful but perhaps someone with experience with both may chime in and be able to tell you how they compare.
From what I've read and heard others say though, they both seem to be very decent, affordable steels and you should be ok or GTG with.