I believe the primary reason FDX has outperformed UPS in the recent past is due to the prior spread in PE between the two stocks. Ultimately, the market determined that the PE's should be closer together. Now, the premium is much smaller, although it is still evident which indicates that the market still likes UPS slightly more regardless of share price.
As for fuel, if it hurts UPS' price, FDX should suffer the same fate. FDX is contending with labor strife in their pilot ranks as well. I do agree that the view of the upside for FDX is brighter but by the same token, there is more downside risk in FDX vs. UPS. The betas of the two stock support this theory.
Short term, my thoughts are the downside risk is stronger for UPS due to the recent run up. I've been amazed how resilient the rally has been in the face of high oil and down market days. As long as earnings and forecasts remain in line, I think the announcement could mean a non-event in terms of substantial price changes. I'd bet slightly to the downside however, due to the recent market gloom. As we've seen, anything construed as negative seems to bring the price down, right or wrong. With high gas, rising rates, and inflation garnering more and more headlines, the mood is gloomy. But hey, everyone's got an opinion so who knows?