Depends what you use it for.
Personally, given the choice between a 3G signal (reasonable internet speed, semi-decent battery life) and a HSDPA signal (fast internet, huge battery drain) I'd go for the 3G. But that is only based on what I use my phone for.
Of course you could say that HSDPA gets things done quicker, so your not on it for as long, so it uses less battery... But in your case the phone will be struggling to keep the HSDPA alive, yet it can get a 3G signal no probs, I'd go for the 3G
Try it and see how you get on, you can always turn the HSDPA back on at any time.
EDIT: The phone will automatically drop back down to 3G if HSDPA drops below a level, but this switchover takes time and power, which was the whole point of the thread to begin with