hard lines are better because they are easier to install/fasten to the frame. all-stainless flex lines are a very expensive option, but my experience is they don't add mush to the system... with that said, I've never had two systems side-by-side where it would be an honest comparison.
I do know that you will have a hard time getting DOT approved all-stainless-flex lines... as was said above, 10 minutes of practice and you'll have leak-free joints. On top of this, there are AN-fitting systems that make crafting brake-line systems a lot easier.
And finally, run away from stainless-steel hard lines. Making them and keeping them leak-free is not easy, and will cause a novice to burn their '40 to the ground. I'll even give you the how-it happens. They make a joint that looks good but the joint wasn't perfectly (and I do mean perfect) deburred. They put it together and it seems to tighten well. They know they cannot overtighten because it will strip the fitting. So they put the system together and it leaks. If they're lucky, it's at an end where they can simply shorten the line and re-flare. Here's where they make the mistake. They don't get a new fitting for the soft side of the joint. Stainless is hard enough that it will cut into the other metal and thus, once it's back together, it will leak.... and that goes on until sanity leaves. I've actually rebuilt someone's stainless system where everything leaked and every fitting and hose had to be replaced because of it.
The other issue with hard-stainless-line is they are nearly impossible to flare on the vehicle because of the force needed to flare the fitting.
On the other hand, as long as you are careful, buy a good cutting tool, and take your time - the steel/stainless flex line system won't leak and will work as advertised.
If you are building a custom system - do spend some time learning about brake line sizing.... wilwood has a great how-to section for this.
