WordPress bookmarklet button UI

In reply to the bookmarklet button, definitely the left. In regards to the overall design of the button then I would be looking at all the types of buttons and there related specific functions then grouping them accordingly. Then creating a style for each button group so you can visually identify that this button belongs to this particular function group etc.

White space is your friend

In most cases white space in your design actually attracts the eye and creates a sense of beauty when properly executed with other page elements.