Hello Treez,
I think 1KHz is a low frecuency to work with ordinary ferrite cores, however, it depends on the density of the magnetic material in the core. Some cores are of Fe powder and others of magnetic materials, ceramic and Fe powder. For low frecuencies, like 1KHz, I think the best option is the second one like your choice (Mn-Zn).
The different values of AL are because the core work is beter at 100KHz than at 1KHz. The "Why" is capacity across the time to mantein the magnetism, in that case, your core has a proportion at 1KHz and another at 100KHz. Ok, I do not want to bore you, in short... For frequencies around of 1KHz, you need to use: 4000±25% and for frecuencies around of 100KHz you need to use: 5760 min. That is, Eg: if you winded 1 turn on the core, if you use it at 1KHz, you will have around of 4000nH; and around of 5760 at 100KHz.
It is not a problem, you can test its work easily!
I hope you can solve your question.
Have a nice day!
Best regards,
Juan_zo