What are the pros and cons of using a grid vs. flat foam roller?
Initially, this is a matter of taste. If you haven't foam rolled before, then it can hurt. In that case, the first one distributes the weight more, and will be less painful.
But if you want to reap the real rewards of foam rolling, then you should be doing it with the gridded one, because it will dig deeper into and between the muscles.
Additionally, when you're more used to it, I highly, HIGHLY recommend advancing to the rumble roller, which focuses the pressure even more for that real massage feeling.
So the only cons you need to consider is pain. If one is too painful, go to a softer/flatter one. But the pros of the grid roller and rumble roller are that they give more reward.