Right when 6th edition was about to hit i switched to IG which has been pretty good to me, and it's great for certain aspects of 6th edition (cheap plentiful infantry and lots of guns).
So in answer to one of your questions... take allies. They fill in the HOLES in your codex which whatever your codex may be even if it just came out it has holes.
My IG have been great with the exception the troops are squishy and hard to hold objectives with and my "assaults" look like a one sided high school girl fight with me flailing my arms wildly and trying to pull out my opponents hair.
The options i've been looking at taking with my IG are...
DA:
Pros: the new hotness, cheap good tac squads and scouts, ravenwing and deathwing
Cons: other than the devestator squad which is the cheapest and best in the game since it gets flakk missiles all the other heavy choices suck (whirlwind is ok i guess and very cheap at 65 points)
BA:
Pros: strong HQs, sternguard!, fast vehicles (if thats your thing) and great dreadnoughts
Cons: dreadnoughts aren't as great as they used to be in 6e, troops are a bit expensive
GK:
Pros: Strong HQs, good troops and elites, dreadknight!, psyriflemen
Cons: EXPENSIVE point wise, really takes away what you can do with your other atachement, not friends with anyone else
Vanilla:
Pros: scoring sternguard with kantor, droppods that fit 12, ironclad dread?
Cons: a codex that is starting to show it's age, expensive HQs, expensive troops
the other thing you need to think about is which is going to be your primary and which is going to be your secondary detachment?
I was originally going to go IG primary with one of the above secondary to maximize IGs power with 3 heavy slots. The more i looked into i realized i would likely do better with a strong HQ from a detachment as warlord and being able to use 3 elite slots from BA or DA for example.
Another thing you should consider when doing your list is being able to do well in any scenario that is thrown at you come tournament day.
More troops are more better... unfortunetly not every codex has strong or cheap scoring units
need enough firepower to kill your opponent of having 6 troop choices isn't going to help
assaults, my IG have taught me i need at least one unit that can punch people in the face
mobility? i know you love bikes and nobody does it better than ravenwing
thats my 2cts, IG is great if you want bodies on the board or big guns. Anything else lots of space marines codexs to choose from