I debated the Sport Line vs Base when I made my order. I ended up ultimately going with the Base model due to the fact that I knew I'd ultimately change the wheels/tires and get coilover suspension regardless of the line. The $2500 I saved ($2280 cost?) I will have to put towards those modifications. The only thing I'm really missing out is the sport seats. I didn't have a chance to sit in the F30 sport seats when I ordered, but the side bolsters didn't look as dramatic as the E90 sport seats so that pretty much sealed it. Not to mention I wasn't a fan of the extra silver trim on the bumper and the red trim line going across the dash.
Now if I was in the boat you're in where you find yourself wanting to trade-in... I can imagine it would probably be at least a $2-3k hit and if your state doesn't calculate sales taxes on only the difference like California... you're in a for a big hurt ($5-8k loss). That's a lot of money for personalization.