In my point of view, your question seems a little vague. If I understood correctly, your are looking for the advantageous of EEGALB Vs. the Matlab signal processing toolbox in the EEG signal processing. In this case, I should say that EEGLAB has been designed for the quick EEG pre-processing usages only. On the other side, available codes in the Matlab's signal processing toolbox have not been only formed for the pre-processing part but also designed for the main signal processing sections.
I would like to recommend MATLAB if you have to learn more about EEG data processing as MATLAB has wide range of functions and you can get different ways of data representation. Moreover, you can extract different parameters specially to present the results for publications. If you need only the specific results, go for EEGLAB.
EEGLAB is a toolbox based on Matlab. so for per-processing, inspecting the acquired signal and removing some artifacts, better to start with EEGLAB. then for further signal analysis you can customize your own code on Matlab