Prospects for high-achieving students are about the same, but the courses and all that comes with it, except for the diplomas, are not by and large the same.
Uni isn't simply about a formal education, and there's a lot that an undergrad experience in law offers that a grad experience doesn't. At the University of Sydney, for instance, law camp isn't available to J.D. candidates, but it is to combined LL.B.ers, and only first years. Moreover, as a LL.B. student, due to the load, you, in theory, have considerably more time to get involved in stuff, be political, or simply screw around. That can't be true for any J.D. student who wants to be gainfully employed.
It's perhaps preferably to obtain an LL.B., combined with something, rather than obtain a J.D. after completing something.
In a vacuum and making generalisations, it seems that the LL.B. can be perceived as being less rigorous or at least intense, as you have your first year law classes spread over three years, then hit law really hard in your penultimate and ultimate years of uni study. In contrast, most J.D. programme start hard and simply stay hard. Some even have intensive pre-O-week or O-week classes.
J.D.s grads may have life experience, having taken some time off after finishing their undergrads; however, there may be some that are simply doing law because they have always wanted to do it (but, for whatever reason, couldn't) or realised that they'd like to give it a go. Experience can be invaluable and improve your employability. Some J.D. grads can be as dumb as a brick, though, and they may not have any experiences on top of LL.B. grads and they may not distinguish themselves in any particular way.
LL.B. grads tend to be younger, more pliable, and, if you demonstrate potential and it's relevant to you, more groomable (in a non-sexual manner). They are generally distinguished on the basis of their achievement and attainment, and most of them are already pretty smart cookies, at least for UNSW, USyd, Macq U, and UTS. Those that do law and seek to enter the industry are probably serious enough about staying in the law for some time. There are always those that do it because of the skills, to become better informed of your rights and responsibility, and other reasons too.
I feel that LL.B.s can better present themselves in terms of academic achievement, part-time work / work experience / clinic, essays / research / mooting / review. It's seldom the case that J.D.s can manage to do or match all, or much, of this. Indeed, it could be difficult for a J.D. to be gainfully employed at a top-tier firm. That's pretty ironic, as a J.D. can be viewed as committing yourself to the law or something close to it. (However, I suppose I am being deliberately obtuse by viewing top-tier practice as the pinnacle to which all law students should aspire. It isn't, but it's pretty generally rewarding, soul-sucking, and enlightening.)
The actual answer to this is "it depends", which is a classically legal way of answering this. That's just my take on this and my two cents. Someone else may present a countervailing, but equally valid, view of this all.