I've been thinking of doing hill sprints/repeats for a few months. I've spent most of this year doing LISS, including running a 10K for charity, and I feel like switching to some HIIT-style training for the winter months. I have two hills within easy walking distance. Both are roughly the same length - around 250 Metres - and grade. The only difference is that one hill is a street, so I'll be running on asphalt. The other is grass covered. Is there any advantage to running sprints on grass as opposed to asphalt, or is the effect much the same for both surfaces?