It's gonna come down to portability and personal preference.
As demonstrated many times on this board, everyone has thier own preference.
For portability, SLA's are kinda heavy to lug around, but they "usually" give you the longest, most stable power. Thier discharge curve is more stable than NiCad/NiMH rechargables and have much higher current levels. But if you're ONLY running a NJB3, you can get away with something smaller than a 7ah ecocharge. I already had them to run my V3/D-10 Pro.
DIY regulators will work, but since the JB3 is very finicky about it's input voltage, the SUREST way for it to work correctly, is to get Creatives Car adapter, and modify it to work on the 12V battery of your choice.
In festi mode, you could run a "fully charged" JB3 of your SLA, and if that started getting flat, or you got concerned about having enough juice left to run your pre-a/d for the duration, you could unplug the JB3 (though not in the middle of recording) and get the full capacity (5-6 hrs) off the internal batteries.
There will be as many opinions here, as there are people running JB3's. IF they WORK, they are all good suggestions. Which one you choose, depends on your budget, your willingness to haul additional weight (in the form of SLA's), your competence (and faith) in constructing an "off-label" DIY regulator/battery combo, and a couple of other factors.
For me personally, I'm comfortable with the internals, in a single band 3-4 hour gig, and running the Creative Adapter off the 12V SLA for longer gigs - but since I'm hauling the SLA anyway, I just use IT all the time.
Your Mileage May Vary...
Rick