I don't know how big the download would be, but as for radio stations, as far as I've understood everything correctly in the DLC you'll get the GTA4 radio stations/songs + the ones added in the DLC. If you get the disc version, you'll get ONLY the stations/songs added in the DLC, PLUS two stations only available on this disc (most notably Vice City FM). So you'll lose some content no matter what you choose, but with DLC you miss out on unique content.
I'd prefer disc in any case - but that's just me prefering something physical over plain digital distribution. --- "You're a jerk, Dent" Wowbagger from The hitchhikers guide to the galaxy