Well, it depends on what you are looking for. If you want a professional website and info dump for people who want to trawl through your long blog posts, find out about your books and where to buy them, then I suppose the blog is handy. But, if you want engagement in a short and snappy medium* then Tumblr is your only man.
*This especially applies to the YA (young adult) market and it applies even more to the American YA book market. I could say that I write for the love of it (which I do), but
if I was to be when I am published I’d also enjoy using my marketing and sales skills to ensure the best possible sales for my book.
I am very new to Tumblr, I still don’t get it, but I can see its appeal. Just look at Maggie Stiefvater on Tumblr. She only set it up a few weeks ago and it is hopping with love. And do not get me started on Sarah Rees Brennan, that lady knows how to Tumblr it.
If you want to suss it out, have a look at mine or even better; set up your own and follow me. We can learn to Tumblr together!