Either would work really well. 1968 usually will have less attenuation and leave you with a sweeter/maltier beer, which (to me) can be good in porters. Also will have some fruity esters depending on how hot/cold you ferment.
1272 would be equally good, won't dry out the beer like 1056, so you still have some residual sweetness, but I think it is a cleaner, less estery yeast compared to 1968.
If I was doing a robust/american style porter I would probably go with 1272. If I was doing more of a british style porter, I would probably stick with 1968. Both are great though. Or you could split the batch and use both =)
Ofcouse, all of these are guidelines, and are not hard rules. You can also influence how the yeast performs depending on mash temp, ferment temp, etc.