>I have the GL package with alloy rims and I am deciding whether to upgrade
>to 15" rims. Is it worth the money to upgrade to 15"?

Oh yeah. Get yer machine off those wobbly hi-sidewall rubber stilts, and
set the German engineering free! Seriously, you will never regret the
upgrade. The handling and stability are greatly improved.

>To get similar rims
>is going to be expensive. What will people pay for my 14" rims?

Well, judging by ebay, looks like maybe a couple hundred bucks?