Its complicated bro, and yes, you're right, Clomid and Nolvadex are similar in the way that they act on the body. However, they are not similar in all of their aspects.
Both of them have agonist and antagonist properties meaning that in some tissues they act as competitive inhibitors in the receptors and in some tissues they activate the receptor that they bind to. The difference between the two SERMS is WHICH receptors they antagonize. Clomids primary function is to block estrogen at the pituatary which signals to the body through the negative feedback loop to create more testosterone. This is important because it will elevate both estrogen and testosterone levels (the estrogen is still present, just not recognized).
Nolvadex's primary function is to block estrogen AT the tissue sites (thus its primary purpose in preventing breast cancer) and only secondarily, and very weakly, blocks estrogen reception at the pituitary. This means that while on Nolva, your body is creating estrogen that is not being received by its inended tissues, but it is also not blocked by at the pituitary. This, in term, means that your body is recognizing that it is producing plenty of testosterone based on your estrogen levels, and it can shut down test production, leaving you with elevated blood estrogen levels and low testosterone levels. The reason you may not NOTICE that your hormones are being trashed is because the estrogen is being blocked from entering the tissues.
Nolva isnt trash for everything, but people dont use it for what its meant for. For men, it is MEANT to treat gyno, and NOT MEANT to be used in PCT. Clomid works very well for PCT as shown by the mechanisms described above.
Hope that helps.