Barry H. . .I feel you're pain in having to own one of those crappy flat grind BG-42 Sebbies.

Relieve yourself of that nasty pain. . .and send that piece of junk to me.
LOL
sygyzy. . .
I assume it is referring to the type of steel used in the knife right?
You're correct. We're talking blade steel.
Not really a major issue. This subject is just talked about alot.
There were slightly over 150 flat ground blades produced. There are some who prefer hollow grinds over other types of blade grinds. There are some who prefer flat grinds over other types of blade grinds.
Those who purchased flat ground Sebbies reported superb craftsmanship, excellent fit & finish and outstanding cutting results.
I don't have the chronological sequense down pat. . .but shortly after the release of the flat ground Sebbie (in BG-42) it was announced that they weren't going to make any more. Thus making the flat grind Sebbie a limited run.
So. . .being the knife snobs that we are. . .

. . .the push began (I think that it was more in the line of whining

)to have CRK make flat grinds in S30V. The push went approx. 1" .
LOL
Does CRK use both and you get a choice?
When CRK began making their knives they used ATS-34 for their blade steel. Some time later BG-42 was considered a better blade material. . .so CRK elected to us it. When S30V hit the street. . .again CRK began changing their manufacturing to use that blade steel.
In any manufacturing process. . .as a company upgades a certain product it exhausts stock on hand (i.e., BG-42) as it's slowly introducing the new material (i.e., S30V).
Were there choices ? There were no choices in flat grinds. They were only made using BG-42.
How does one differ from the other?
S30V was made strictly as a blade material. In fact, Chris Reeve help create the steel (with Crucible Steel). Both BG-42 and S30V are very tough steels. S30V is tougher, retains an edge longer and doesn't have the edge chipping issues that BG-42 has.
I know that I'm missing all of the nitty gritty details while explaining the difference between BG-42 and S30V. . .but the above were the major issues.
Yes. . .as with any product. . .the new & better stuff is preferred.
Watch the other knife makers (not so much low and medium production outfits) and you'll see that they are producing knives with S30V blade steel.