Here is a general plan to start debugging this problem. Post your results and we'll help you figure this out. I'm running an iMac with Snow Leopard and these steps have helped me.
Write down key data for each machine for each wifi connection. I've included my data below.
Go to the iMac and hold down the option key while clicking the Airport icon in the menu bar. Write down the PHY, BSSID, Channel, RSSI and Transmit Rate.
Move over to the MBP and gather the same information.
Phy Mode this tells you if you joined the network as a B, G or N connection. I'm on an Airport Extreme N network using the 2.4 Ghz frequency.
BSSID - This is the access point you are logged into. They are likely the same on each machine. If they are different, your machines are logged into different access points.
Channel - This is the channel each machine is using. In my case, my neighbor and I were both on channel 1. I changed over to 11 and my speed improved since I was no longer fighting his interference.
RSSI - Received signal strength. Mine is -43 while my neighbor's is -88.
Transmit Rate - The higher the better. Mine is 130. I can't view the neighbor's rate but it's like less due to his lower RSSI.
If you see other nearby networks, you can view some information about their signal by moving your cursor over the other network and a small dialog box will pop up with their PHY, BSSID, Channel, and RSSI. This is how I found my neighbor was using the same channel as me.
You can also view this data in System Profiler under Network>Airport.
RSSI or Received signal strength indication is what is used for the display of the bars.
The higher the number ( closer to 0 ) the better your signal strength.
For Apple devices they used a scale of -100 to 0. But you'll never see 0, or even close. Most people will see a number between about -50 and -80, with around -50 being excellent.
-50 will show all the bars and -90 will show gray bars. Not sure if it will go to -100 since you don't have a signal then ;)
So if you're asking for a scale I think its around 10 to 12.5 RSSI per bar.
While you have a good signal strength, your throughput speed ( Tx Rate ) isn't that fast.
One method to calculate signal quality is to compute the Signal-to-Noise Ratio (SNR). SNR is the signal level (in dBm) minus the noise level (in dBm). Both of these values are typically represented as negative numbers. For example, a signal level of -53dBm and a noise value of -90dBm would yield an SNR of 37dB (i.e., SNR = Signal - Noise = -53 -(-90) = 37)
The calculated SNR value, as measured from a wireless client, would decrease as the range to the base station increases due to applicable free space loss. Also an increase in RF intereference from microwave ovens, cordless phones, walls, ceilings, etc, which would increase the noise level, would also decrease the overall SNR value.
SNR Guideline
40+dB = Excellent signal
25dB to 40dB = Very good signal
15dB to 25dB = Low signal
10dB to 15dB = Very low signal
5dB to 10dB = Little or no signal
In your case -65 - -90 = 25 or -70 - -85 = 15
So you have a 15db to 25db signal OR as diagnostics calls it SNR which isn't that good but isn't really bad either.
Best Answer
Excellent work with the details. The transmit rate and MCS index indicate that your mac has a high-speed connection to your router and that it's not an interference issue.
My next step would probably be timing me file transfer using Wi-Fi from the Mac to the PC or vice versa. You can enable filesharing on the Mac quite easily. download some YouTube movie from the Internet so that you've got something that will take 15 to 30 seconds to transfer so you can time it with the stopwatch.
It's hard to guess whether flash that runs the speed test.net simply running slow or you've actually got a legitimate speed issue with the Mac.
You'll also want to watch the activity monitor and the console application to see if you have io errors and if the network throughput matches the disk io which should all matching your benchmarking test.