Should I pre-built rafter units, or build them in-place?
Since you're putting a gable roof on it, I can only assume this is being built for storage purposes, not for aesthetics.

Building it with a glulam ridge beam would be more difficult, but it would be much stronger.  The main issue with a ridge beam design would be lifting the beam into position.  If you can manage that pretty easily, the rest would be a breeze.  I'd put some ceiling joists up at either end, lift one end of the beam over one of the joists, pushing it far enough to clear the ceiling joist on the other side, like so...

B
   B
      B                 
  JttJttJBttttttttttttttJttJttJ     
  S         B                 S        
  S            B              S
  S               B           S
  bbbbbbbbbbbbbbbbbbbBbbbbbbbbb

B = ridge beam
J = joists at the ends of the barn
S = studs at the ends of the barn
t &amp; b = top and bottom plate


... and then lift the bottom end of the beam up with some ropes.  The joists will need some strength to support the ridge beam, and you'll need to be able to stand on them, so I'd use three 2x6's @ 9" OC on either end (where the outermost "2x6" is actually the top-plate of the walls on either end of the diagram) with some 5/8ths plywood on top of them.  I'd toss in some temporary studs at the center point to make sure the joists hold up to the weight of two people and the ridge beam.  Once the beam is in place, put in the ceiling joists and plywood on top of them to make walking around easier (and to create an attic for extra storage,) then put the rafters up.

Grab some metal brackets for the joists, rafter heels, and rafter/ridge connections (it'll cost you an extra $125, but it will make the structure a lot stronger) from a HI store and your little barn thing will be able to withstand some nasty winds.