Am mulling over a purchase and while this obviously won't impact it hugely, I'm curious as to how the two stack up. I'm using a 1Ghz 12" PB and quite frankly the reception has never been great; I'm looking forward to a better antenna.
I think the alum. casing hinders the reception a bit in the pro lines. My MB picked up several networks that my MBP cannot, from the same place in my house. But, overall, the MBP's reception is great for my own wireless, and is loads better than my PB used to be. haha, I didn't even know how bad my PB's was until I got the MB.. I couldn't believe all the networks the PB had been missing!
It's kind of a shame that the aluminum hinders reception--you'd think it would turn the entire casing into a massive, laptop-sized antenna capable of picking up signals even SETI misses... but no.
One factor is the casing. An aluminum casing is going to block out some signal that the plastic won't. So a MacBook/iBook gets a whole lot better reception than a MacBook Pro/Powerbook.
Also, if I remember right, the antenna is positioned differently in each computer. The MacBook antenna should run up the left side of the LCD, I'm not so sure of the MacBook Pro antenna.
If you sit with your back to the base station, the antenna will be able to get the signal better at least for a MacBook. The signal won't need to pass through as much of the computer, so it'll be stronger.