Advantages of not topping: Easier to manicure ... Good way to do side by side experiments using plants with different growing mediums, nutrients etc. ...hmm...
I think if your growing for yield, topping, LST and SP (selective pruning) are your bestest friends, topping is particularly god if ur taking clones anyway haha. All these methods do tend to require longer veg times tho, coz the plant takes a lil longer to adjust to having its head removed and such lol.
As for the cola question, that's strain dependent, some strains will have one big cola normally, then if you top it it'll have two half sized colas (added together to be the same as one large cola), however some strains when topped will grow two FULL sized colas, thus, doubling your yield. NOTE: If you find a strain that does this (even better if its a good pheno), then top away!!! I remember seeing this one plant, grown on the roof of this guys house in California, had 16 tops through topping and a little training (LST), every top FULL SIZE, it was huge, and in one fukn huge pot too haha! Wasn't a hugely potent strain unfortunately and one of these Indica dominant skunk hybrids that have become SOoO popular coz of their high yields and short flower period, I prefer good old sativas myself, but this plant was impressive as hell none the less. I wish I had the patients to do that haha.
Cheers!