Way back when AE first came out, MacWorld magazine had a comparison. I vaguely seem to recall that 11b had the edge, but the most striking finding was that 11g dropped off faster than 11b. To put it another way, the throughput of the 2 cards degraded less for 11b as you got farther away. My personal experience with Cisco Aironet 350 11b and Buffalo Airstation 11g wireless pc-cards in a Ti-400 (the internal 11b airport card was worthless, as far as range goes) under Tiger and Panther was that they were roughly equivalent, with a slight edge to the Aironet. In other words, when I switched between cards and measured the connection with stumbler software, the numbers were ever so slightly better for 11b.
That said, your question is a little strange since you can't replace an internal airport card with an AE card? Unless you're wondering about PMCIA cards? Perhaps if you rephrased your question, you'd get a more specific answer?