I did the same trip of Munnar & Kodai during May 2011. I took the following route.
BLR - Hosur - Salem - Karur - Dingigul - Take Dindigul-Theni Bypass road (NH 45) - Bathlagundu - Turn right - Kodai Ghat road - Kodai - Around 460kms.
Absolutely bliss to drive till Dindigul. After that its a single lane road.
In Kodai Ghat road, some sections the boulders were missing and these private buses/mini buses just dont give space. Please provide or halt much before these guys come too close.
Stayed in Kodai International. Pretty good property with nice Kodai Lake view. Its walkable distance to coakers walk and lake. There are lots of restaurants near by as well. There is a good BP bunk at the kodai circle as well. Otherwise next bunk you will get only in Theni.
Kodai To Munnar:
Kodai - Bathlagundu - Theni - Bodi (NH49) - Poppara - LockHeart View point - Devikulam - Munnar - Around 125kms
In Munnar we stayed at decent homestay away from Munnar town near Adimali Road. From Kodai to Munnar there are awesome views you come cross and you wont regret the drive. All the way roads are pretty good. Particularly nearing to Devikulam. You will find decent restaurants if you are non-veg.
In Munnar, there are lots of options in hotels as well.
Munnar - Marayoor (sandalwood forest & Crocodile sanctury) - Udulampatti - Dharapuram - Kangeyam - Erode Bypass - Salem (NH 7) - Hosur - Bangalore - Around 450kms
The other route you have mentioned from Sathy is good as well with less traffic.
You can do BLR - sathy - Coimbotore - Pollachi - Palani - Kodai Ghat road - Kodai
Compared to NH 7 its little empty traffic and less eatouts as well. Anyday, NH 7 should be good for such long journeys.
Hope this helps. Enjoy your journey.
Try to drive to Top station fro Munnar if you have time. BTW
, how many days do you have for this trip?