OK, will tackle this one. I have worked from both castings and forgings since a much younger me machined both exotic-metal pump castings for chemical plants and 3500 pound forgings on a turret lathe with a 42-inch chuck that became the main shaft of drill rig draw works. More recently, I have used both as raw materials for tomahawks ... the emphasis here is RAW MATERIALS.
A lot of folks are familiar with investment castings as the spray painted heads (as-cast, often with unfinished cutting edges) on mass market tomahawks in various alloys sold both by mail and at Rendezvous. These usually have pretty rough hydraulic-pressed unfitted handles. Likewise, at Rendezvous you will see a lot of forged-head hawks in various degrees of quality ranging from "That sucks" to "That's a pretty decent tool." Again, the makers consider these to be "finished" in the rough-forged condition, with perhaps a minimal grind to create a sharpened edge.
Quality makers like Ryan Johnson and Ed Caffrey agree with me that either a forging or a casting is a way to get an otherwise formless chunk of steel closer to the final shape you want it to be. The casting or the forging is RAW MATERIALS to be machined and ground to the final tool. Forgings are older than castings for edged tools, but castings were used in Renaissance times for things like cannon barrels. Nowadays, precision investment castings provide the materials for everything from rifle receivers to airplane parts. In a lot of cases they are used with minimal or zero machining, as for many applications the modern casting techniques produce a part that is already at final dimensions and surface finish.
A quality forging or a quality casting will give you good metal to work with. An investment casting is much more uniform (in quantity) and closer to the "finish shape" of complex objects and is thus more ideal for something like a tomahawk head with its many intersecting curves and angles. If you're going to make just one of something, the forging is obviously the way to go. If you're talking production of multiple items of the same design, however, investment castings are the only thing that make any sense. In either case, you still have to machine and grind and finish the metal to make the tool, but each material source has advantages for a particular end goal.
In tomahawks, there is no way I could make a tool as precise as what I produce and sell by using forgings. They are just not uniform enough, and my super-precision edges result from the use of special tooling that I developed which requires a fixture to hold the head as it is being ground. I used to use forgings for some hawks, and had to "eyeball-grind" the cutting edge (which I can do quite well, but no way as precisely as I now finish ALL my hawks). I also had to reject an increasing number of forgings for layer separations and other fatal flaws, and all of them finished "rough" compared to hawks machined from the investment castings. The castings give me much more control over REPEATABLE quality over multiple hawks of the same standard models.
The castings that I work from, however, limit my ability to produce "unusual" shapes for custom orders for tomahawk heads. Recognizing the limitations of my materials and my own failing ability to put enough hours in the shop to keep up with orders for the "standard" items I finally quit even trying to make custom hawks with one-of-a-kind head shapes. If I were younger and healthier I would use forgings to make those heads ... castings are for multiple heads of the same model(s) though I often machine more than one size from the same casting.
Either way, you are dependent on the quality of the folks that forge or cast the head you are going to work with. And there is a big variety out there.
Hope this information is useful to you.
TWO HAWKS
http://www.2hawks.net