What I was thinking when I was in a similar situation, is how important it's that the user fills the other form fields. If they are optional and the grid is more important I would have the submit button in the grid but if I want the user to fill all the fields, the button should be at the end. In the case where the button saves the whole form, you can have a type of warning if the user skips some fields or you can have a deactivated button.
The second thing I thought was the height of the page. If the button hides 'above the fold' there is a high posibility that some users won't see it and the data won't be saved. So in that case, maybe you should have it in the grid. I would check analytics about screen resolutions.