Both, in that WP on your own domain gives you maximum flexibility as to content, functionality and design, while Blogger gives you faster blog creation, indexing and ranking. You could create a WP blog then start a blogspot as one of the Web 2.0 properties linking to it, thereby getting both indexed and ranked faster. Or, you could start a blogger as a free testing platform for the niche you are working on, then move it to your own domain when you see the niche is indeed working.