I don't have any experience with a 7' cutter but I was talking to Dad the other day who does. He claimed that their old single spindle 7' pulled as hard as their newer twin spindle 10'. I have no reason to doubt what he says.
I agree, go with the 8', unless you've got a deal on the 7'.
If it gives you a point of reference, I'm running a 6' with a DX33. Have to go slow when it gets really thick or woody....but she pulls her just fine. Yes....front weights were necessary. lol