We had a similar decision to make last winter and ultimately went with concrete. For us, the look of concrete goes better with the house and it's cleaner because there is no sand between pavers to track inside. That, and it was the cheapest option. Wet-laid flagstone and brick are much nicer but about 5x the cost.

Depending on the gravel, there may only be minimal surface prep needed before being able to pour right on top since you're already 4" deep. Especially if you go with concrete, make sure you get someone who knows what they are doing and gives you details on exactly what will be done (rebar spacing, type of concrete mix, finish type, cut lines, etc).

It's true that concrete cracks. After 1 harsh winter we have a few _hairline_ cracks (you really have to look to see them) but even so we're still very happy how the slab turned out.