Well I have never been a huge fan of 20 man squads. When it comes to close comabt, you might get those extra guys in there but rarely will you have enough room/time to use all of them effectively. I'd go with 2 squads of 10 instead.
And yes a skull champion is worth it. With the extra attack and options (give him a power fist) you'll have a model with 4 attacks on the assault with a strength 9 weapon!