DPH is hydrophobic and is often used to monitor the hydrophobic core of the bilayer. TMA-DPH is believed to monitor the more shallow regions of the bilayer (glycerol backbone and upper segments of the phospholipid acyl chains), the cationic group present in TMA-DPH is supposed to act as an anchor to the water/bilayer interface.
See for a recent comparison between the two and relevant references:
do Canto, A. M., Robalo, J. R., Santos, P. D., Carvalho, A. J. P., Ramalho, J. P., & Loura, L. M. (2016). Diphenylhexatriene membrane probes DPH and TMA-DPH: A comparative molecular dynamics simulation study. Biochimica et Biophysica Acta (BBA)-Biomembranes, 1858(11), 2647-2661.