They're different, but equal in terms of benefits--do both. Variety is important. You could also do some fast running over a rolling course. Keep one of your weekly speedwork sessions and replace the second with the hillwork.

Not that I'm an expert-but I have read A LOT about hill training (specifically Lydiard's views) and "how" you run the hills is very important...I used to find a hill and charge up it as fast as I could numerous times...then I figured out that hill repeats are all about power and not so much about cardio (or course they can be)...hill springing is what Lydiard called them, the idea was that you would climb the hill with a high knee lift and good arm swings, and in a slow, bounding type motion, work your way up the hill. He also emphasized using your ankle to "spring" upward...this was NOT a workout to leave you exhuasted cardio-wise. And the hill work was done 2 to 3 times early in the training cycle and maintained throughout at least 1 every two weeks as goal race approached (marathon for me)...Obviously hill running can be done many ways-including hard running up the hill which will put you anaerobic rather quickly...this type is best saved for the end of your training cycle when you are developing your anaerobic engine. Hope that made sense...good luck!

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"Fortitudine vincimus"