It sounds, from your usage, like you don't need the extra niceties of the Dice unit. If all you want to do is play the iPod through your stereo using a high-quality input, and you're on a budget, the least expensive way to go sounds like the best.

Unless you need it to recharge your iPod's batteries, another option is a BMW Auxiliary Input cable, which attaches to the back of your head unit. They're about $75.