General Trade Value of D vs. O in IDP leagues?

[rockchalk]

I have played in IDP leagues before and currently play in a 20 team IDP/PPR dynasty league with 5 player "practice squads" for players with less than 16 NFL games played.

On offense we only start 1 RB and 2 WR with no flex spots. On defense we start 2 DL, 3 LB and 2 DB.

My linebacker corps is full of injury risks. I am currently starting Merriman, Barnett and Tatupu with Keith Rivers on the bench. The best two options in FA are Quincy Black and Jarret Johnson (or Joey Porter if you have faith in a resugence, which I don't). I have AP as my starting RB, J. Stewart on the bench and Spiller on my practice squad.

In general or in my specific situation, what are your thoughts on value of offense vs. defense when trading. I've got a lot of interest in Spiller and the best offer I've got is David Harris even up for him. They are projected to score the exact same number of points in our scoring system (280). I just can't get a feel for what is a good deal when comparing the two sides of the ball even up.

Any experience or insight would be appreciated.

Cheers.