I think mode based by album seems reasonable. Include the entire catalog. Plenty of sound clips, etc. I'm almost thinking like Time Machine like.
Have the ability to temporarily remove songs you don't care for.
As to GNR vs LZ pinball,,, we'll have to wait and see. Music wise? I like both but LZ has the Blues background I can really appreciate (albeit stolen in some cases they really laid down some nasty tracks and renewed a classic sound).
Much like ranking drummers/guitarist/etc is useless,we must accept each band as their own. Certain things appeal to some and not others.
No artwork for LZ? Album wise perhaps, but I wouldn't let that deter me from original artwork if it works well.
Toys/considerations:
Stairway
Blimp
Gong
Swan Song
Symbols
GNR lightshow seems top notch, and I hope the game is great, but that alone won't hold people for long, so let's hope it's fun too.
Stern I'm certain isn't worried about which machine will be better, only that theirs sells well. I believe they have learned from The Beatles fiasco.