I would recommend the FIVE. I started end of last year and still leveling my first team, and right now, I cannot ever see myself one boxing again. Five felguards plus 15 dots on a mob or another player (five fears on round robin, that poor player or mob) is just too evil and so fun.

Plus, you can do all five man instances yourself. No more qq and other hassles.
Tank, 3 dps plus heal...Holy trinity + two. 'Nuff said.

It all boils down to what you want to do. Five would be the best as you open up lots of opportunities. With 2 or 3 boxing, you still need two or three others to fill the 5-party for instances, unless you overgeared and outlevel the instance. As you said, five lightning bolts are awesome!

SaraiE