A testing tool's efficiency depends on the number of virtual users it can accommodate on a single test operating device.The efficiency is more if your business requires fewer testing devices to produce a larger-scale virtual test. So it must be efficient enough in generating an expected number of virtual users on the existing hardware.
Cost is another important factor when choose the right performance testing tool. You could check and review the the tool's licensing before using it. If it is a paid tool, check if the cost of the license fit within your company's budget and choose the right one within your budget.
The performance testing tool you choose needs to be able to access enough hardware and network resources to produce a sufficiently available performance testing environment. If the tool can't generate a test environment that simulates the expected amount of traffic, the tool may not meet your company's needs.
If you choose performance testing tool which is not easy enough to use, it may cause your IT staff more problems than they solve. The right performance testing tool will cut down on time spent learning the new software which will implement on your company's platforms.
The support from the vendor is also very important. You could ask what the level of user support you can expect from the tool's vendor while you need have problems. Generally, vendors offer high-quality support through different channels for the customers.