I'd say if anything, the card would probably be faster, since it's tapped directly into one of the PCI slots, as opposed to a hub which hooks into one of the computer's USB ports only to travel even farther until it reaches the hub.
Another pro of a card: it doesn't use up one of your existing USB ports.
One alternative is to get an Apple monitor or another brand that has the USB hub built in, this way its sort of the best of both worlds. I have an older 21" CRT Apple monitor with the 4 port USP hub integrated and it works wonderfully well, no extra desktop clutter and the cables are easy to reach. And even though the new LCDs only offer 2 extra ports, its still a good solution.