This was kind of a tough one for a minute as I think the R34 is better looking than the A80 and possibly a bit faster as well. However, the Supra is RWD, so more pure, and it was available in USDM form, which means it's more patriotic to drive a Supra than a Skyline. Thus, scrap the Skyline and take the Supra.
Ok, a more serious answer, since you didn't specify which era are both cars, i'm gonna assume it's any year for both cars, i will scrap the C110 Skyline and take the A40 Supra.
id kill the skyline and replace it with a 2004 Subaru Impreza STI --- Post updated --- eclipses are good cars sadly they get riced out with terrifying bodykits gross airbrush and stancelife stickers and non functional carbon wings heres an example