I can certainly echo what Ben has posted, above. Theravada can mean different things in various contexts. There are many posts here on Dhamma Wheel that discuss fluently the variations in Theravada.
As for your descriptions of Thanissaro Bhikkhu, and that of Ajahn Brahm, I can offer my own opinion. I'll be brief, as to answer fully would take me more time and thought than a Saturday morning will allow.
I have attended Wat Metta where Ven. Thanissaro is the Abbot. He teaches from the Pali Canon, and primarily from the Suttas and Vinaya. He is seen by some as "orthodox," and perhaps humorless ( at least as compared to Ajahn Brahm), but I can tell you that he is a terribly interesting person, with a keen sense of humor. He has a huge smile, and enjoys the environment and people at Wat Metta, especially the Thai people that are part of his lay sangha. He's strict with his young monks in training, but also fatherly to them. My view is that he has spent his teaching life trying to get the meaning of the Buddha's teachings via the Suttas as right as he can, and then tries to pass along that knowledge the way a concerned parent or coach might try to teach their charges. Rather than life being banal and meaningless, I feel he tries to illuminate the possibility that life is rather amazing and has unlimited potential, but only if we are willing to suppress our defilements and do the work necessary to achieve release. To use another analogy, if you were an athlete trying to make the Olympics would you want a negligent coach that joked and farted around, or a coach that would teach technique, encourage you and at the same time, give you a kick in the arse when needed to move you forward?
As for Ajahn Brahm, my view is that he is a solid Dhamma scholar, and a dedicated teacher. He has incorporated humor as a means to impart his teachings. He has been very effective in reaching a large audience, both in Australia, and worldwide, by bringing the Dhamma of the Canon, as well as the commentarial texts, to a large mass of people that otherwise would have had little interest in Theravada Buddhism. Because I feel he reaches out further from the Suttas than does Ajahn Geoff, Ajahn Brahm has a different take on certain subjects, such as jhana, where he teaches a commentarial type (VM) deep jhana vs. a Sutta jhana as does Ven. Thanissaro. Ajahn Brahm also quite capably talks on subjects that come up in lay life and psychology, and integrates Dhamma with psychology, sociology, physics, so as to make these lessons applicable to people in distress. He has more of an engaged approach ( ie prison ministry, engagement with politicians) than does Ven. Thanissaro, who seems to stay somewhat clear of what the west would call "engaged Buddhism."
There is the Theravada Sangha of Thailand, and one of Burma, and Sri Lanka, Laos, Cambodia, et al, and of the west, and in each culture you will find differences, but a large body of sameness. You will find, as in the case of Ven. Thanissaro and Ajahn Brahm, two different kinds of teachers teaching from essentially the same textbooks. I might suggest doing what Ben alluded to, and what I have tried to do, is to seek out as many competent and ethical teachers in Theravada that you can, and spend the time over months and years evaluating these teachers, and seeing which ones resonate with you and with others that you trust.