BJT's Small signal Models
You can use either model to analyze a small signal BJT circuit, but the hybrid-pi model is most commonly used. The T-model is supposedly easier to use in certain situations, but I've always used the hybrid-pi model without a problem.

The main difference between the two models is that the hybrid-pi model uses \$r_{\pi}\$ and the T-model uses \$r_e\$. Here's the relationship between the two:

$$r_{\pi} = \left( \beta + 1 \right) r_e$$

where \$\beta\$ is the gain of the transistor. On datasheets this may be \$h_{fe}\$.