I have seen the review result by AV-test about the two security app. Although they basically have the same ratings on protection and usability, the only factor they differ is cost and features. Both of them are also award winners in different categories by AV-test, but Kaspersky has the most award between them. When it comes to cost Kaspersky is much more expensive than Bitdefender. Now if i where to choose between the two, ill go for Kaspersky just because of the ratings and awards it has gained from AV-test which is an independent testing institute.
Kaspersky definitely, by far. It's basically the worlds best antivirus software. Because it's so popular, there are the most updates, so it won't lag your phone or cause any trouble.
Also, it does the best job at protecting your phone. Just stick to it and you won't have any problems. I am speaking from experience, with best intentions