It's not really fair to ask a bunch of die-hard Blackberry users what other device they'd rather have.
But since you asked, between the two I'd have to say the Q, but it's definetly a case of the lessor of two evils.
The only reason I'd go with the Q is because it would be a major mistake to get the 8125 with the updated version (the Hermes) being released soon. Also, the 8125 is not a "smartphone" but a PPC where as the Q is a smartphone. They run different operating systems.
However, realize the Q is only available with Verizon. It will not work on Cingular. So if you do go with the Q you'd have to change providers.