oh silly 4000...... it is the same damn core but clocked slightly higher... same core mind you... if you picked up a 4000 and a 3700 and OC'd them both on the very same board the 4000 may go further, the 3700 may go further... it depends on the revision of the core and oh, i dont know, the bloody weather on the day it was made
if you want to know which of the 2, i say 3700 because uhhhh its cheaper... if you want to be smart about it... check the stepping of both the chips... pick the best stepping... there are certain sandys that will go beyond 3ghz at stock voltage AND on air... but there are some that crap out a little past 2.6 no matter how much juice you feed them.