Well come to the First and Last line of Defence of Man Kind.

Seninels are not a troops they are Fast Attack in the
FoC (odd that since thay an't really that fast). The battle box can be used to make 1
CCS and 2 Vet squads, the heavy weapons can then be used (or not) in them. It gives you a legel army in a box (with some tatical flexabilaty with heavy load outs), more than most other battle boxes do.
Your right about the
IG Platoon numbers, we can out hord even the most hordyist of armys with just 1 troop choice. as for cost, yes the
IG do get on the pricy side after abit, hell just getting a 1500pts foot slogger list is going to cost a hell of alot of cash (but painting it is going to take longer to do)

Again, welcome to the ranks solder!
As to which to use, well I went with the Cadians as they mached the
IG look for me