I would say it depends on some different conditions. Bugs are VERY expensive to build as an army, but can look very cool once placed on the battlefield. On the other hand, my 3000 point bug army is freaking BRUTAL to transport. If you've got to use a motorcycle or public transportation forget about it!
The MI are equally cool looking imho, and I feel they offer a greater variety of playing styles/strategies. You can also put together a very effective force for a relatively small amount of money, and fit the same force in a container the size of a small shoebox.
I don't know anything about the Skinnies beyond the S&P preview, but as I'm the only non MI player in like a 300 mile radius AND I'm tired as hell of lugging those bugs around I will most definately be joining the ranks of the Skinnies next month!
Bugs are way cool and not as expensive as some tend to think. If you have friends in the area, then a couple of box sets will give you 40 warriors, which is a good start. Add a tanker, and 2 boxes of fliers and you have a decent mid size force. Tigers and Guards can either be painted in different schemes, or if you like modeling Guards can gain added size with some conversion work. I plan of coverting to make my King Tanker with a longer and wider body, a extra set of fore legs, and longer back legs.