In one of those weird twists of coincidence, I went on some subject leader training today run by Sheila Eastwood, and we ended up discussing  you guessed it  exactly what I blogged about yesterday, and how this fits in with the idea of mastery in mathematics. We pretty much concluded as I did yesterday; at the moment, noone really knows what will be allowable as a method on the new GCSE papers.
So in my followup post, I thought I'd look at pros and cons of all three methods on the assumption that pupils are allowed to use any in the new GCSE exams, and then consider implications for only allowing long multiplication at the end. Any claims I'm making here are backed up by absolutely no proper research and are mostly anecdotes from my classroom, but I can't seem to find much independent research on multiplication methods.
1. Grid
Pros
 Cons

2. Lattice
Pros
 Cons

3. Long multiplication
Pros
 Cons

Besides which, if we're talking about "traditional" methods, lattice wins handsdown. There's a good reason it's got so many different names (Chinese, Napier's, Gelosia, I even found someone on YouTube calling it "lettuce" the other day...), and that's because it's one of the oldest forms of multiplication we have, far predating the apparently historic algorithm favoured in the new primary curriculum.
P.S. A little Googling indicates that at least one person agrees with my stance on lattice  see this blog post on CavMaths.