There are quite a few differences between the two types. If you don't have a license for either one, I would go with Garamond Premier, because it is more cost effective. You'll also note that Garamond Premier is more lively, whereas Sabon Next is quite sterile. Visually, Garamond Premier is more interesting - that may or may not be a reason to choose it, if you are looking for something more bland, go with Sabon Next.

My own choice would be Garamond Premier, although I have not seen what exactly is being set.

Thanks for your response.

Yes, the price difference is startling, which is why I would get only four styles for Sabon Next.

It's a scientific text for university, done with TeX. A classic appearance is important.

I've tested a friend's Sabon Next, but I couldn't get the leading right (tested 13pt and 13.5pt, either to dense on the first page or too sparse at others).

You mention that you will be setting formulas. I would check and see in each of the font families includes all of the glyphs that you would need to set before making any decisions. This might eliminate one or the other off the bat…

Yes, I thought about that. I guess, I'll be using the MathTimePro fonts for formulas.

Arno alo works well with tex. In particular, you could make use of the optical sizes for captions etc. Warnock or Minion are also options. The MinionPro latex package supports pretty much everything.