They should be more precise thanks to the external x-over. Whether or not they'll be louder depends on which speaker is more sensitive. I'd bet that they'd probably be able to reach a little deeper than the 6x8 triaxials.
BTW, 6.5s are just as easy to install as the 5.25s, so I'd probably go with the larger for a lower frequency response up front. I'd bet they're probably a bit more sensitive too, which would be a definite plus running them off the HU.
