wifi speeds

why is the Airport wireless connection consistently slower than that of a PC?

I assume that you mean, why is your MacBook Pro's wireless connection consistently slower than that of a PC ... correct? Also, are you referring to the wireless data transfer speeds between the MacBook Pro and the wireless access point or the overall data transfer speed between either the Mac or the PC to a remote Internet site?

sure, you assume correctly (though looking around the Internet would suggest slow wifi speeds on macs are not unheard of...)

No doubt, because of some network configurations or the clients themselves, you can have scenarios were one computer performs different ... but this is not unique to any one vendor's product. For example, I have a consumer-grade Internet service, rated at 50 Mbps download. All of my network clients, which include a mix of Macs, PCs, and Linux boxes, get roughly the same "speedtest.net" results ... actually between 45 & 60 Mbps. The differences are more related to how they are connected (wired or wireless) and current Internet traffic conditions rather than the actual networking equipment used.

Is there a way to quantify the data transfer speed between the MacBook Pro and the wireless access point (other than saying better or worse than an ethernet connection) that I could try?

Yes, try jPerf for Mac.