A very good (and up-to-date) reference article on this topic can be found here: Tim Dettmers Which GPU for Deep Learning.
Because MATLAB's Deep Learning Toolbox uses CUDA and CUDNN libraries just like almost every other deep learning framework, the recommendations in the article above should mostly apply for MATLAB as well.
Please note that the opinions and statements in the article belong to the author and are not formally endorsed by MathWorks.