Raindrops on roses and whiskers on kittens: Daemon/Witchhunters

Daemonhunters

Pros:

• Wargear list lets you really customize your army
• Officio Assassinorum (nuff said)
• Can ally or be allied with Witchhunters, SM or IG

Cons:

• Some of the stuff is very "vs. daemon"-specific
• Can't cross-ally (except with Witchhunters)
• Inquisitorial retinues are pretty weak. Paper tiger syndrome.

Witchhunters

Pros:

• Wargear list lets you really customize your army
• Officio Assassinorum (nuff said)
• Can ally or be allied with Daemonhunters, SM or IG
• Sisters of Battle (Space Marines with boobies!!)

Cons:

• Some of the stuff is very "vs. psyker"-specific
• Can't cross-ally (except with Daemonhunters)
• Inquisitorial retinues are pretty weak. Paper tiger syndrome.

Summary: As a standalone army, both of the Inquisitor armies are pretty beasty, but lack a lot of flavorful supporting units. Sisters of Battle, especially lack much in the way of anti-armor (unless they ally). Grey Knights are pretty much limited to advance-fire-charge, not a lot of "interesting" units, although theoretically they could ally with space marines to fill in some of the gaps. I think both army lists together might have some potential (especially with the right fluff), or possibly as allied units (perhaps doing some sort of "counts as" fluff) to an IG or SM army.