I don't think you have mentioned how much money you plan to spend. If you have unlimited funds, Rivendell makes several models of bikes that would be ideal for your needs.
If I understand your needs, you want a bike that will easily carry 300 pounds of rider plus cargo. You want to be able to use it for both commuting, (which means riding in the rain sometimes) and for longer week-end rides. So, that means a bike with rims designed to use tires in the 32mm to 38mm range, with a fork designed to leave clearance for fenders.
In 2006/2007, the term "road bike" has come to mean "racing bike". And, a true "racing bike" is NOT designed for 32mm tires plus fenders, nor is it designed for 300 pound loads.
So, you need what today is called a "touring bike", such as the Trek 520, or a hybrid bike, such as the Trek 7.3FX. And, a mountain bike, equipped with light weight slick tires, would also meet your needs very well.
A good touring bike, hybrid bike, or mountain bike is NOT inferior to a "road bike", although most members of "Bike Forums" think of road bikes as being somehow superior, because 1% of their owners use them for actual racing. What makes a bike a "good bike" is how well it matches the actual needs of its owner.
You don't plan to enter the Tour de France, nor are you likely to attempt to ride one hundred miles in less than four hours. You need a bike that is tough, durable, and comfortable for five or ten mile rides, even when dealing with bad roads, rain, sleet, and all the other stuff commuters face. And, if a bike can carry 300 pounds of rider and cargo under those conditions, it is a very good bike indeed.