There's no reason you can't do both, and get the benefits of both. If you are in a phase (or week) where your priority is getting in several faster speed sessions, cranking out two hours or more at 15 seconds over marathon pace is perhaps not the best way of being fresh for 400s two days later. That is the time to take it pretty easy -- you will still be getting a lot of aerobic benefit from throwing in the one long run a week. Pure Lydiard.
On the other hand, if you are in a phase/week where you want to concentrate on aerobic development running some of your long run hard can be very good for you. Opinions on how hard is hard vary -- some people would recommend that you don't go much faster than marathon pace, others would say that you can throw in portions at half-marathon pace. But you need to think of these harder long runs as a workout day in itself, which you may want to take an easy day after.
There are all kinds of variations on this, 60-90 minutes easy, followed by 30-60 at marathon pace, etc. Or start easy for 60-90 minutes, then move down through marathon pace to maybe finishing at 10 mile pace, or even quicker.
Long runs with hard sessions can be subtly tiring. You can feel great the next day, and not realize how tired you are until two days later.