If your a GIS pro, then the tower "gps" found on your phone is even better than survey-grade GPS. You can get to a few feet or less if your in a box of towers. If, however, your doing survey work then always buy the GNSS equipment if you can afford it.
Space programs are running almost exclusively through Russia now, and as the other countries get their systems up and running GNSS will keep on getting better while GPS stays the same or degrades as old sats break down or have to go off-line for whatever reason. Also as better reference surfaces come available they may require access to the GNSS network to be useful. Not having the right equipment in the near future may count as negligence on your part if you don't use the correct reference and state what your system is looking at.
If your really want to find a good answer, see if the CORS in your area are referencing GNSS and GPS. If your on a cellphone based reference then your already using GNSS and need the hardware. Some dealers may be able to upgrade your current system to accept GNSS code, I don't know if they do that kind of thing but it would be cheaper than buying a new system.