[Aerodyne]

Hi Ben
The name 'Lacacote' seems to suggest that it is in fact cellulose-based. Who knows these days! I suggest that its a case of horses for courses - if you are definitely going to French polish an item, perhap the shellac product is most 'authentic'. If however you intend to lacquer with cellulose or similar, the cellulose sealer might be the one to choose. It's worth saying though that either can be used for any finish as, as far as I am aware, shellac sanding sealer is unaffected by cellulose - and vice versa.
The only reason I tend to prefer the cellulose sealer is its ease of use: shellac products are a little messy for the less-than-professional worker. As Col says, wear cheap hand protection or you'll have the devil's own job getting the stuff out of your fingernails.
Everything has its vices, though and cellulose products are highly volatile, so beware of the fumes if, like me, your lungs are somewhat compromised.
Neither type of sealer is primarily intended to grain fill but the shellac type will do so given patience. Grain filler can be bought in tints as a paste, but it is messy stuff in my experience. I've seen French chalk added, as Col mentions.
-Tony