Generally speaking it`s better to switch to 7700 (workload permitting), as it makes clear to ALL controllers (not just the one you`re talking to) that there is an emergency in that sector.
If you can picture what a busy ATC sector or facility can be like, this fact alone means other controllers or sectors will be less apt to bother the concerned controller with other items that can wait. Additionally, if the controller did not hear your `Mayday`or `declaring an emergency`for any reason (other traffic talking over you perhaps) he or she will certainly see the immediate and obvious 7700.
The controller will let you know if and when the 7700 is not longer necessary for ATC purposes.