I tend to use a very old program called IPERF to test lan connections. The default setting just trys to see how much tcp data it can push though the connection but it has a number of other options that can measure things like jitter.
It will not tell you the signal strength but signal strength is only a part of the issue since you can have good signal levels but get poor performance if you have lots of interference. Still whatever device can push the most data is still the best choice no matter how many bars it says it has.
You are best served to have one of the machines on the wired ports of the router it makes the test much more valid. Running multiple wireless signals causes lots of random derogation so its hard to get good test results.