I feel that this is a forum is for discussing shaving, not simply directing people to look things up on Google. So, I will actually answer your question! It is essentially the same head as the HD. It doesn't take long to get used to the bigger handle and I find it to give a nice shave.
Its long handle has a good heft to it too. Its also known as the Barberpole
This was my entry razor into wet shaving. Its an integral part of my rotation.
I believe they do in fact have the same head. Being somewhat on the large side of normal, I bought a 38C as my first razor because I thought that the other razors looked too small (understandable, coming from M3). I love my 38C, and I use it regularly, but I have learned that the size is really not an issue. I enjoy shaving with my little 40's SS just as much and maybe more.
It is a great razor, but if you already have an HD, and it is working for you, then there is probably no need to get a 38C.
Having owned a 38C at one time for close to two years, it was one of my very favorite razors. I sold it a bit ago and grew sorry about that decision. Just purchased one today on the BST ( I do these types of things) and look forward to having it again in the shave den. The Merkur HD is also a great razor but I personally got my very best DE shaves from the 38C. I had sold it because I am primarily a straight razor shaver now, but I missed it.
I don't think you would go wrong buying either the HD or the 38C. The head geometry as been said is the same but the weight and style of the 38C gives it the "edge" in my book. Whatever you choose, enjoy! (and WELCOME to B&B!!!)
Yes, that was his warm welcome to the forum. His last activity here wasn't much later than that. I hope he comes back but wouldn't blame him much if he didn't.