I guess both will be reality quite soon. The successful MD-11F is history now and all pax-MD11s will be converted eventually (half of them are already freighters or are planned to be converted in the near future), so the similar-sized 777 would be a perfect follower. Its a wonder to me that Boeing didn´t lauch it already.
A330F is another issue. Maybe the A340 makes a better freigher?
Airbus has already begun offering an A330-200 freighter to compete against the B767-300F. It is possible that it will get launched at the Paris Air Show this summer. It is aparently recieving quite a lot of intrest. The A330-200F was introduced to the public at the begining of the year. I doubt that the A340 would be that desierable as a new-build frieghter, it's costs are higher, and nonstop flights are not that crucial for frieght operators. And at any rate, the A330-200F will have a range of at least 4000nm, comparable to the 767F, so it will be a trans-Atlantic cargo carrier. Unlike the A300F that it to an extent replaces.
Boeing is the company that carries out the conversions for MD-11 pax to frieghters. There are still lots of Md-11s in pax service, so maybe Boeing doesn't want to hurt it's own operation, as demand for used MD-11 will only fizzle once thier all gone, or there's a better alternative. That's where a 777F could fit in.
New-build freighters are very expensive, and there are only a few airlines that can afford to purchase them - ie Fedex, Atlas, Cargolux, UPS. The A300-600 is having a great "second-life" as a pure freighter and at some point, I guess when the A330 order backlog begins decreasing, we will see an A330 take over this role. A 777F is still a few years away, I think: 747 and MD-11 conversions are still more practical and economical plus, there will be many, many 762/763 aircraft in future available for conversion.