I always seem to just grab the biggest impact wrench I can, but sometimes that's not what's needed. For smaller jobs in tighter areas, the M12 3/8" Ratchet really shines. With up to 55 ft-lbs of torque, it should make light work of the toughest nuts, including the ones on those darn Corvette headers.  If you had to choose one, I'd probably pick the 1/4" version, but it nice to have the extra torque.  Just know that the head size of the 3/8" and 1/2" versions are bulkier and don't fit in as many places as the 1/4" does.

My order of purchase would likely be: 1/4" Fuel Extended, this 3/8" Fuel Extended, 1/2" Fuel Ratchet, 1/4" Fuel Ratchet, and finally the 3/8" Fuel Ratchet.  I know many like the regular, non-fuel version due to the smaller head size, but I never find myself using those.   

Specifications:

• Model: 2560-21 (2560-20 Tool Only)
• Fastening Torque: 55 ft-lbs.
  
• RPM: 0-250
• Anvil Type: 3/8" Ball Detent
• Head Profile: 0.76"
• Length: 15.1"
• Weight: 3.1 lbs.