Which attack takes place more often over the internet? I would have assumed DDOS due to the amount of botnets around today.
I would say that DoS attacks are significantly more common, I should also point out that a packet flood is not the only form of DoS / DDoS attack.
For example, I know of a fairly popular php product ( a Video encoding / gallery product, not going to name names since this is still undisclosed) which even in its current revision can be massaged correctly into fork-bombing the server that is running into oblivion.
A DDoS requires a botnet or multiple machines to do this so the barrier to entry on this is higher.