The two are the same; the author deliberately differentiates between them.
태자빈 is a non-standard (and technically incorrect) word; it just means 태자비 (= 황태자비) according to 우리말샘.
As you know, 후보 means candidate(s) and nominee(s). Thus, "태자빈이 아니라, 마치 황태자비 후보로 온 것 같군그래." could be translated as "It does seem that (you) came here not as the Crown Prince's wife but as a candidate for the Crown Prince's wife." It may also indicate that to be the prince's wife, she should behave in the way the duke expects a princess to; in other words, it does not seem that she is suitable to be the prince's wife. It seems that the duke will not recommend her because she behaves improperly.
Since this question seems somewhat irrelevant to usual usages of words, I have searched the Web and skimmed over a lot of pages. You should have given the title of what you read. I assume that you read the second volume of "버림받은 황비" because I have found this there:
경에게 하는 양이나 아내 후보 운운하는 꼴을 보아하니, 태자빈이 아니라 황태자비 자리를 노리고 온 것이 분명하군.
(If it is not the case, please mention the correct title and quote the full paragraph in your question.)
This clearly indicates that the author deals with the two words differently. I have noticed more hints in Volumes 1 and 2 whereas you said "There isn't much clue." I wonder whether you did read the whole contents of these volumes.
The protagonist (모니크) says in Volume 1:
(...) 저는 어떤 형태로든 황태자 전하의 비가 되고픈 마음이 없습니다.
(...) 저를 태자빈, 즉 미래의 황비로 삼으려 하시는 것이 아닙니까.
황태자의 비 means the wife of the Crown Prince. 미래의 황비 means the wife of the emperor in the future. Basically, the author uses these phrases to mean 태자빈; however, the meaning changes a bit later.
She says in other parts:
저는 황태자 전하의 약혼녀, 이니까요.
약혼녀 means fiancée, which indicates that she will be his wife in the future. Some other paragraphs say that she does not want to be his wife, though.
And she is said in other parts:
황태자비.
그 지위를 한 단계 내려 태자빈으로 삼는 것이 안전하겠지만, 네 가문이 그동안 바쳐 온 충정을 보아 그리하지는 않겠다. 되었나?
The author differentiates between 황태자비 and 태자빈 here. In this novel, 태자빈 is a lower position than 황태자비. The narrator (actually, the protagonist) adds:
정비의 지위를 유지해 주는 것. 그는 내가 후비가 될까 두려워한다고 생각한 모양이었지만, (...).
Thus, the author regards 황태자비 as 정비(正妃) and 태자빈 as 후비(后妃); she/he thinks that 정비 is higher in position than 후비. If dictionaries are correct, using 정비 and 후비 is technically incorrect because 황태자 is not the king. Standard Korean Language Dictionary defines relevant terms as the following:
정비: 정실인 왕비를 후궁에 상대하여 이르는 말 (queen who is 정실 as opposed to 후궁).
정실: 본처 (아내를 첩에 상대하여 이르는 말; a word used to mean a wife as opposed to a concubine).
후비: 임금의 아내 (wife of the king).
후궁: 제왕의 첩 (제왕's concubine).
제왕: 황제와 국왕 (emperor or king).
정궁: 황후나 왕비를 후궁에 상대하여 이르는 말 (empress or queen as opposed to 후궁).
A princess is said in Volume 2:
왕녀께서는 태자빈 후보로 오셨음에도 장차 황태자비가 되실 모니크 영애께는 아무런 인사나 양해를 구함도 없이 황태자궁을 들락날락하셨습니다. 아닙니까?
This indicates that 모니크 will be 황태자비 whereas the princess is just a 태자빈 candidate. It also implies that the princess attempts to be 황태자비 in place of 모니크, so in the novel, 황태자비 and 태자빈 are different in position. Now we can say that part of LJLee's answer is correct. Therefore, "태자빈이 아니라 황태자비 후보로 온 것 같군그래." means "It does seem that (you) came here as if (you) were a candidate of the primary wife of the prince not that of his secondary wife." It is understood as "(You) came here in order to become the secondary wife of the prince, but I think (you) are trying to become his primary wife."
In conclusion, 황태자비 means the main/primary wife of the Crown Prince whereas 태자빈 compared with 황태자비 refers to his secondary wife or concubine, in the novel that you read.
As you might have noticed, people do not always use words as defined. In numerous books, you can even find words coined by the author. I believe that a careful aberration from the fixed definition is an attempt to express something efficiently and effectively.
editbelow the tags (if you are on a PC)