I assume you need to add circuits.
If there is room in the main panel for additional breakers, that should be considered first.
You need to factor labor and materials to get the additional circuits back to the main box.
A J-box will need EMT or PVC pipe from main panel to it, then circuits pulled,
(derating if there is more than 3 current-carrying conductors) all splices made up, neutrals, grounds, etc.
With a sub-panel, you'll have to run one heavier cable back to the main box,
but you'll use less branch-circuit wire,(as your 'homeruns' will be shorter).
The biggest mistake made with J-boxes is people put in ones that are to small,
they think a 4" box is enough, when it's far from it.
If you'll be branching out with many circuits, get a big one, 8 x 8, 10 x 10,etc.
I've seen horror-shows of boxes that look like a plate of spaghetti w/wires stuffed in, 1" long screws used to "pull" the cover down.
And the minute you dig-in...BZZZZT, now your stuck fixing the short-fall.
Usually the labor to install a box with individual wires is equal to adding a subpanel, or close.
Depends on how many circuits are involved, the distance, and the difficulty getting between locations.
Sorry if I ranted there...
Last edited by Do It Right; 03-30-2011 at 06:13 AM.