Honestly since you're comfortable asking the question of how to balance these, you'll be fine :) Arguably they are simply two sides of the same coin. GD is framed around the experience someone gets in the old browser; while PE is framed around the newer browsers.
As a philosophy PE works really well as you focus on that simple, universal experience and only add things where you actually need to. GD takes a bit more discipline in testing and it's easier to miss something, but it is very effective all the same.
It can help to look at your site's usage statistics, to see what most people are using - ie. do most people have the good new browsers, or do you have a lot of people on old gear? Do you have a high proportion of mobile vs desktop? It can be a good way to inform your decision and know where to focus your time/attention.