Code 75 is certainly much finer. Apart from the rail height, the points and flex track in general are more flexible and take shaping better (as the rail is less substantial).
Trade offs include the fact you need to replace or re-wheel / bogie any older stock and that the tolerances are generally tighter, so its easier for stuff to derail.
On the pro side, its a lot more prototypical (and less trainset) looking, its easier to lay and curve.
On balance tho, if you've already a lot of 100, I'd stick with it. You can also get peco widgets to let you transition from 75 to 100. So you could maybe buy 75 concrete track for the mainline and use the code 100 in the sidings or yards?