Difference of performance bewteen a core i5 and core i7?

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I really don't know, but in general I would guess that more threads are especially valuable on a multiuser machine. The sage notebook server has been undergoing some recent changes, and I expect it would make good use of the additional threads. So I would say it's worth the extra cost, especially with a view to do more things in the future, but of course it's not my money . . .

You could probably get a more expert answer on the sage developer list: sage-devel (you do have to request permission to join, but it usually takes less than 24 hours to go through).

If you plan to take exams and/or work with novices, I'd say it's worth it. It's not easy to jail each process and give it limited resources. If X students write an infinite loop, you server will collapse. The more threads and RAM you have, the greater X can get.

If you divide the available RAM into the total number of students, you're wasting resources, it's better to estimate the number of students that can write an infinite loop that appends to a list, and not hit interrupt and/or restart on time.

In my experience, the probability of a student blowing the server is higher on a exam than on a regular class, and it's higher in combinatorics or number theory, average on linear algebra, lower on calculus (unless the exercise involves symbolic integration of non-trivial functions).