Do colleges prefer the SAT french subject test score or the Delf B2, which is a test issued by France to test for foreigners’ French, evaluating reading comprehension, writing (about 250 words), oral (listening and speaking).
For most colleges, the answer is neither. For those few colleges that recommend Subject Tests, then those colleges want Subject Tests, not DELF (or CILS/DELE/JLPT, etc)
As I am sure you know, DELF tests at a higher level that the Subject Tests, so if you scored well, you can certainly list it in your additional information. Note, however, that no college is going to ask you to “prove” your proficiency level for admissions purposes.
A good SAT subject score in French will be helpful - it’s about tested at CEF A2- and a good DELF score on top of it will be excellent, but if a college wants to see subject test scores the DELF can’t replace that.