I say go for both: have a small standing army of 2 dwarf melee squads (so they spar a lot and stay happy) and draft everyone but the miners, hunters and woodcutters into permanently inactive militia squads, preferably equipped with bone/wooden crossbows and bone (for now) bolts. Settings: wearing uniform when inactive, carry any drink, carry no food (food from backpack doesn't give dining room happy thought). Danger rooming the standing army to skill levels where they don't bitch about duty anymore would be useful, but dangerous without metal armor- but bone and leather might suffice for that. Shields are mandatory for everyone, and can be made of anything.
Before you draft people full-time, make sure they have good attributes (can be improved by a screw pump gym, but the danger room will boost them too, except regeneration and disease resistance) and are at least Happy, but preferably better. Before you draft me full-time, I could make some leather armor, or if we don't have much leather, keep it for waterskins (those are a must-have for everyone, saving time) and quivers.
Have one designated bone carver so he can be trained up with plenty of masterwork totems and doesn't get melancholic about a masterwork cat bone bolt hauled off-map in a goblin's ass.
We will need a meat industry for all that leather and bone. Ask the Outpost Liaison for a breeding pair of dogs and pigs, both are non-grazers and don't adopt owners uncontrollably.
As we don't have metal armor, taking on the first ambushes would be suicide. Let's just kill or capture them with traps, take and equip their metal weapons and armor.
As for trap design, I say do ALL the suggestions!
http://i3.kym-cdn.com/photos/images/newsfeed/000/187/321/xallthey.pngHave the top few levels dedicated to all the trap layouts. The trade depot should stand in a big room empty of traps, where the army will be able to meet siegers on it's home turf, and the help of any caravan guards present at the moment. This room is surrounded by airlocks: raising bridges at all entrances, and an unarmed half-goblingrinder (citizen-only entrance, build this first!) leading to the fort like this:
W-wall F-floor X-stairway H-hatch cover over empty space P-pressure plate linked to hatch cover D-door B- raising bridge C-cage trap T-weapon trap
WWW WW
FFFFWWWW
XXXFDHPBFF
XXXFWWWW biiiig room with depot where hostiles are killed, and can be sealed off to recover wounded and goblinite safely.
XXXFDHPBFF
FFFFWWWW
WWW WW
From that big room, many passages with different designs (with raising bridges on both ends, open each as the situation demands) open towards the surface. Mine input is a short 2-tunnel goblin grinder with cage traps on the fortress side and weapon traps (with whatever melee weapons we have at the time, or menacing wooden spikes- 2 traps with 2-3 trap components per tunnel will suffice.) on the other, and a longer zig-zagged 6 tile wide (to avoid human-dwarven caravan collisions) path for wagon access, with shared start and end drawbridges. Close both in case of building destroyer attack, let other trap corridors handle it.
FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF
WWWWWWWWWWWWWWWBBBBBBWWW
FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFW
FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFW
FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFW
FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFW
FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFW
FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFW
FFFFFFWWWWWWWWWWWWWHWHW
FFFFFFFFFFFFFFFFFFFFFFFFFFWPWPW
FFFFFFFFFFFFFFFFFFFFFFFFFFWCWCW
FFFFFFFFFFFFFFFFFFFFFFFFFFWCWCW
FFFFFFFFFFFFFFFFFFFFFFFFFFWCWCW
FFFFFFFFFFFFFFFFFFFFFFFFFFWCWCW
FFFFFFFFFFFFFFFFFFFFFFFFFFWFWFW
WWWWWWWWWWWWFFFFFFWFWFW
FFFFFFFFFFFFFFFFFFFFFFFFFFWFWFW
FFFFFFFFFFFFFFFFFFFFFFFFFFWFWFW
FFFFFFFFFFFFFFFFFFFFFFFFFFWTWTW
FFFFFFFFFFFFFFFFFFFFFFFFFFWTWTW
FFFFFFFFFFFFFFFFFFFFFFFFFFWPWPW
FFFFFFFFFFFFFFFFFFFFFFFFFFWHWHW
FFFFFFWWWWWWWWWWWWWBWBW
FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF
FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF
FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF
FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF
FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF
FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF