[R] generalized linear mixed models - how to compare?

[Nestor Fernandez]

Dear all,

I want to evaluate several generalized linear mixed models, including the null
model, and select the best approximating one. I have tried glmmPQL (MASS
library) and GLMM (lme4) to fit the models. Both result in similar parameter
estimates but fairly different likelihood estimates.
My questions:
1- Is it correct to calculate AIC for comparing my models, given that they use
quasi-likelihood estimates? If not, how can I compare them?
2- Why the large differences in likelihood estimates between the two procedures?

Thanks,

Nestor