Good question, but it's harder to do, because I want to test in the same environment. In theory Flac will always be better, but how noticeable to someone like me who's attended too many punk concerts without hearing protection? I dunno. Spotify can do up to 320kbps MP3 on the phone/computer, which is quite a bit better than Slacker was streaming. But we have no control over how it runs through the car's player. If I were to play it through BT on my phone in the car, it goes through further compression in the BT-passthrough, so you're losing quality there, too.
Either way, I think if you're driving around with windows open or on the highway, the quality of Spotify should be plenty acceptable.
