No single strategy works for every business or situation. Your unique choice of implementation depends on several factors such as:
Organisation Size: For midsize firms, a quick implementation strategy often works best where the software is implemented throughout the company at the same time while letting go of the old system instantaneously. However, this approach may be too risky for larger companies, and a phased rollout extending over weeks or months may be better, though more costly.
Risk Tolerance: If users face major issues after deploying the new system, how well will the firm be able to handle this situation? Suppose the business is entirely reliant on its systems operating 24/7. In that case, it may be best to keep both software running parallel to each other and slowly migrate area by area to avoid a crisis despite the running costs.
Cost: If the project cost is a considerable concern, it's best to go for quick and efficient methods such as deploying software all in one go, as it eliminates the costs of running double software.
Desired Pace of Return on Investment: With a phased rollout, the benefits of a newer, more effective ERP system come one by one as you adapt to it area by area. With a full implementation in one go, you have the potential to reap all those benefits at once when all users are using the same integrated system.