Thanks for your question! It seems to me you should not have identified your career aspiration as you ask for "career prospects" among the 2 options...but I guess you should know they are not comparable in terms of job scope, work projects and industry salary benchmark. It's hard to say which one has a better "出路" as we don't even know which position of "digital marketing" you will be engaging...say in-house or agency? Sorry that we may not be able to share an initial thought with you.

The role of a tax position itself is not useful for you to get into advisory / consulting, but the role in Big4 may help you to get an entrance / access to get known with ppl inside, including campus recruiters, managers or even partners, which should gain you some value for chasing for an advisory role. Afterall it is not a matter whether if it helps, but whether you have other internship offers to choose.

Lastly, I guess you may want to take a moment to really think about your career interest, say if you find digital marketing much more attractive than big4 roles? Or you may want to start by re-thinking from Day 1 of your university life - why you chose Marketing as your major? Is it something you are with passion? I would highly suggest to choose a path that you have interests / at least you can enjoy at least a moment of working on that side.