Checkthis certainly has a beautiful UI/UX. I like the fact that the add-content bar between blocks doesn't interfere with the block spacing (With Sir Trevor the add content bar adds height between blocks), and the grouping of blocks in the add menu of Checkthis is going to work better for a larger number of block types than the Sir Trevor horizontal list.

The drag-and-drop repositioning is also much slicker with Checkthis, as you get to see the other blocks shuffle around the one you're moving, rather than partially so with a copy of the block.

Sir Trevor is a good start though!