I would like to use twitter but for driving traffic you must have targeted twitter community. You can use hashtags and tags persons for viral your tweet.
The two choices are very great and execute their responsibilities very well, but to get a personal taste... I select Twitter because it is more helpful for me everyday (I can acquire information and fun in precisely the exact same time).
I would suggest to go for both, Improve your impressions which ultimately improves your sales.
It all comes down to the market that you're targeting. In my opinion, tumblr have a lot of bot accounts which is why i try to avoid tumblr for my business.