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reason

The key is in the chemical used in the respective wheels. The SG us aluminium oxide which is suitable for carbon steels which is most knives and hand tools. The SB wheel is silicon carbide which is harder than aluminium oxide and so is better at sharpening alloy steels which are too hard for aluminium oxide. I am talking abut scratch hardness are used in mineralogy. What you are using to sharpen has to be able to scratch the tool or knife being sharpened.

If you have hard alloy steel knives such as Japanese knives that are glazing your SG wheel then you will need to use something harder to sharpen those knives. Either buy the SB, ur hand sharpen using a silicon carbide stone or diamond hone.

Thank you for that clarification but I guess that leads into a follow up question.  How would we know what (Rockwell C) hardness a knife may be?  It is possible that we could look up the manufacturer data sheet to get this information but I think I most cases we are presented with a knife with the request "can you sharpen this?" but otherwise know nothing about the technical details of how it was manufactured.

I think I know my own answer but am looking for validation.
Currently I have the original SG-250 but have seen good videos of the SB-250
My understanding is that the SB-250 may work better on really hard steels (are any kitchen knives in that range?) and is also a bit more aggressive/fast than the SG-250.  But if time to sharpen a knife is not critical (I am retired and only sharpen a few knives for friends and family) then the SG-250 is perfectly fine.

Basically I am looking for a reason to buy the SB-250 in cases where the SG-250 is inadequate

I find that the SG-250 returns to an average 600 grit after a couple of knives. If I am resharpening a knife I have previously done I will use this mid grit then use the stone to get near 1000 for finishing before honing.

How long it will stay at a particular grit is also related to the pressure you apply but it won't be too long.

Thank you, that makes sense and I will pay a bit more attention to using the stone to get back to 1000 after a few knives

I have the T8 with the SG-250 stone.  Assuming I keep the stone grade at the nominal 1000 grit, how long will this last before it gets clogged with steel particles and will need to be regraded, or maybe even resurfaced?

First off I am new tormek T8 user and new to this forum and thus apologize in advance for any etiquette violations.

I wonder if the tormek SJ-250 Japanese water stone is worth it.  One concern I have is that with use, the standard SG-250 will decrease in diameter and will become smaller than the presumably lessor used SJ-250.  Thus when we sharpen on Sg-250 at 200 then 1000 grit and then change wheel to the new SJ-250, the diameter will likely be different enough that we are no longer simply refining the previous edge but will in fact have to reshape the edge according to the new 250 mm diameter SJ-250 wheel.  This seems to imply we might was well just do all the grinding on the SJ-250 which is clearly not the intended use.

Am I overthinking this issue?  But it seems to me regardless how we use the AngleMaster, in the limit the metal edge (chisel or knife) will have a radius according to the SG-250 wheel (lets assume 200 mm).  Then when we attempt to finish the honing with the 250 mm SJ-250 then the chisel front and back edges will touch the stone but the center will have a small gap.  We would have to continue grinding using the SJ-250 to completely hone to the new 250 mm circumference.