Personally, I like the existing mantel better than the proposed one. While the tile surround could be changed to something else, I don't prefer either of the proposed photos you've posted. I think it looks odd to only have the surround on between the top of the fire box and the mantel vs. how it runs down the two sides next to the legs of the fireplace. It also looks odd that the "slab" is split into two pieces.

Also, what does your fireplace insert require re: clearances around firebox? City codes tend to follow manufacturer's specifications re: clearances. Most require a minimum number of inches between the firebox and combustible surfaces.