As a rule of thumb, any activity that is using the screen is just as bad a making a phone call via 3G. So a minute of phone use is the same as texting with the screen on.
If you are using iMessage, it has a connection going with 3G too. So that is worse than plain old SMS texting. Not Apple's fault, but is true of other 'free' texting appls that use the data connection. If you are using iMessage, trying turning it off, and just use sms, for comparisons. Doesn't make a difference if you are imessaging or not, just being on, will suck more battery.
FYI, I have been getting normal battery from my iP4S, about 7 hours of usage.