In the current unstable market, the general advice is, it is better to sell first so you know how much $$$ you have to spend.
Be prepared though and be actively looking for your new home, in the streets and locations you want so you are ready to act when your sale occurs.
Make sure your home is ready to sell, that you have had a professional valuation done and have decluttered etc. Maximising your homes selling appeal.
Ask for a generous settlement time, 120 days to give yourself time.