Bugs, because you can always find people who want to play MI, and the arachnid models really can't be beat. Plus we get Tanker and Plasma bugs, which makes me feel like I'm playing an Armored Company or something.
Back when I was playing D&D, I always liked to be the DM controlling the monsters. Now playing the Arachnids I can still be the scourge from which the MI players can save all of humanity. Somebody has to be the bad guy, and I've been told I'm a pretty bad guy.
I collect both 'cos I'm the only player around here.
But I think the bugs are cooler, and by and large, better models than the MI (excepting that piece of !@#$ called the brain bug).
The basic bug soldiers in the boxed game are good models, very easily posable, easy to get good painting results from and and look great when painted.
The MI... I have been disappointed with the static and limited range of poses of the MI in the boxed game. I see the other kits (heroes, etc) look better, but with 32 MI in my two boxed sets, I am using them before I buy any of the specialist MI like veterans, female MI or heroes.
I go with the Bugs. It's soooo much fun to see the look of dismay on the MI players' faces when, after they set up their handfuls of troops, I start laying out masses of warriors & friends, then say, "Not done yet, I have to get the other box out of my car."
I have made a chas bot out of a griffen from the mechwarrior dark age game and I am going to play test it today. The MI have an awesome chance againts the bugs if you play disaplend(I thik thats how you spell it)on the battle field and think hard about what the bugs are trying to do.