What grade(s) are you considering starting with?
I have registered my oldest two kids with Kolbe from K (they are in 6th and 8th now), and have used some Seton Materials. They are both very good options. In my opinion, Kolbe’s greatest strength is its flexibility. You can follow their curriculum exactly, or you can substitute as many of your own choices as you wish (up to 100% changes) and still receive a transcript from them. Obviously, total substitution might indicate you don’t really need to be registered with Kolbe, but the idea is that they take the principle of subsidiarity very seriously.
Kolbe also offers science that is more comparable with a rigorous science curriculum you’d get in building school. They use good, secular, non-offensive sources, and add pertinent religious perspectives in the lesson plans. They also offer a non-Saxon math option, which is also on the more competitive side of things. Personally I love Singapore Math, so I like that they have added this to their elementary curriculum.
A plus, or minus, depending on the student, is that Kolbe uses original sources for high school history. My ds, who is a very good reader and who loves history, is going to be well served (I suspect - haven’t started high school yet) by that approach. My dd, OTOH, who is not a strong reader and who doesn’t enjoy history, has had trouble with Kolbe’s history, and has done better with Seton’s history books.
While I tend to trust Kolbe’s book choices a bit more to provide a balanced approach to history (Seton is very heavy - maybe a bit too much so for my taste - on the Catholic perspective), Seton has a very nice series of History books that are not written at an advanced reading level, and which have been wonderful to use. Seton gets a bad rap for being “dry” and “workbooky,” but sometimes the most simple approach is the easiest and most effective one. That has been my experience with my dd. Depends on the kid, and also on your teaching preferences.
I am told that Seton really gets kids writing well. I’ve heard about “the dreaded book reports” that I think are expected to be written very specifically. My impression is that the writing that is expected is extensive and frustrating, but that the results are good. I have not heard as much praise of Kolbe’s writing, and in my experience, it has been one of their weaker areas. While they do suggest a lot of writing in the lesson plans, they don’t give much direction on how to compose paragraphs, book reports, and essays, so has been harder for me to teach. I have ended up substituting other curricula for writing.
I don’t know how well either school teaches reading, as I used a different program for learn-to-read. But their readers in the elementary grades are also quite different. Seton’s readers look like dick and jane picture books from the 50’s. Many find them quite charming. Kolbe uses the Catholic National Readers from the 1890’s. The layout of the books is not as nice, and the writing style can be archaic at times - however I see that as a positive aspect, because it really challenges students to work hard at reading, and I think it prepares them well for the more difficult and older language they will have to read in high school.
I could go on and on… what works for one family might not for another. I personally dislike the middle school writing program that Kolbe uses, but I know families that love it. What has worked for my son in history, has not worked for my daughter. For all I know, I will end up with her doing Seton in High School… I have also found that in the earlier grades, affiliation with a curriculum provider has become less important. I don’t register my younger kids with anyone right now, and I just do a mishmash of things with them. Some Kolbe materials, some Seton, and some other.