I would say keep the old one, unless it is less than Class 6 speeds.
If you buying a decent enough phone where you need good read/write speeds (like capturing in RAW or recording in UHD/4K), better to go for the latest HiSpeed cards.
Another point to note is that the old card, if it was made into Adoptable Storage in the old phone running Android Marshmallow, then its better to leave that card there. It is encrypted and will not be recognized in any other device.