Tumblir is for me a wealth of erotica whereas Pinterest is a place to collect pics of all sorts of stuff. Pinterest displays a whole lot of stuff at once and is really easy to search whereas tumblir you sorta need to look at each pic individually . I like Pinterest for keeping a visual perspective on everything I find esthetically pleasing or self defining but tumblir is where I come across super sexy awesome photography everyday .
For blogging it would depend on what type of blog. I would do tumblir for an adult blog because Pinterest will flag anything pornographic but people can follow just one of your boards on Pinterest as opposed to the whole thing anyway - why not do both?