the results from 3dLME seem better
In what sense is the 3dLME result better than 3dttest++? Do you mean that the two approaches show a similar pattern, but 3dLME tends to provide somewhat stronger statistical evidence?
Regarding the difference between models with "fixed" versus "random" effects: Results from a model with random effects can be generalized to a broader conceptual population, while results from a model with only fixed effects are confined to the specific recruited participants. In other words, the model with random effects offers much more powerful interpretability.
In general, conventional approaches for adjusting for multiple testing (e.g., the -clustsim option in 3dttest++) are often excessively conservative and tend to overly penalize the results. You can find a more detailed discussion in this blog post.
Gang Chen