To echo much of what has been said, I tend to favor BBTS (at least at first). However, more recently I have been splitting almost equally between BBTS and TF Source.
To be clear, you can also choose to pay later at TFSource for preorders. I did so very recently for my MP-13 Soundwave. One thing TFSource does better is the free shipping on orders over $150 and since MP Soundwave is $169 (same at BBTS) it was better to go with TFSource for the free shipping. I also picked up AM-12 Breakdown from TFSource while he was sold out on BBTS. I even preordered the Silas verson from TFSource while he was still sold out on BBTS too.
However, I buy all of other non-TF stuff from BBTS. Customer service has been great for both. I had a preorder question about MP Soundwave and asked TFSource and BBTS the same question on the same day. They both responded 1 day later.
So, it's really a wash because their prices are usually the same. But if you're spending over the $150 mark, I'd go with TFSource for free shipping! I love them both.