Yes the schools do have specialisms so one appeal will be a lot stronger than another I think. However the key is the extension capacity of both schools especially as they both have sixth forms. Our ds is currently doing year 8 maths work and the comprehensive offered (without being negative) does not have a sixth form and has limited capacity to offer extension in some subjects. Hopefully we can evidence all of this.
The Y8 maths work and the fact that he is working at such a high level could be mentioned in the letter from the primary school, if you are arguing that the school appealed for will meet your son's educational needs. You should be able to find out what kind of extension facilities are available in the school offered vs the school appealed for. Anything you present needs to be fact-based and objective. If the "limited capacity to offer extension" is backed up by comments in an OFSTED report, for example, then that's fine but you also need to prove that the school you want can
offer this kind of extension (e.g. maths clubs, subject-based visits and events, etc.).
Is it a common occurrence or rare do you know to get the same panel ?
I know that the same panel has to hear all oversubscription cases for the same school, and I believe it is possible for at least one member also to appear on the panel for different schools. I expect they are supposed to treat each appeal as separate in its own right, but I imagine it might be awkward since you know that they know...if you know what I mean!
A similar situation cropped up last year but I can't remember who it was - have looked on the sticky but still can't find it. They won both appeals and were then faced with the problem of which school to choose!