I've been on both Delta One and VS Upper Class within the past 12 months. Here's some comparisons:

Seat: On DL, I was on both the 767-300 (9 rows of D1) and 330-300. On VS, I was on the 330-300 with 1-1-1 seating. The seat on Delta is easier to operate and is a bit more comfortable overall, although on VS there is a mattress pad in addition to a covering. DL involves placing your lower legs and feet into a "footwell"; on VS, the seat is at roughly a 45 degree angle to the window, and your feet are open.

AVOD: DL generally has a better system with more programming choices than the "Vera" system on VS, although on our trip which involved ATL-MUC-ATL, the video touchscreens didn't work properly (and we were on different aircraft for each direction).

Food: DL's food was not very good; my wife was disgusted with what she was served in both directions on the MUC trip. My filet of beef on the return was as tender as a block of wood. On my VS trip LHR-JFK, my filet of beef was quite tender and delicious.

Beverages: Delta had both Woodford Reserve bourbon and Grey Goose vodka in miniatures. VS serves hard liquor in J from full bottles; its American whiskey was Gentleman Jack. It too serves Grey Goose. Delta's wines are supposedly hand picked by the Queen of Cuvees, the Princess of Plonk, the Master Sommelier herself, Andrea Robinson. VS claims to get its wine selections from London wine merchants Berry Bros. & Rudd (the firm on the label of Cutty Sark). I think both airlines were serving Lanson champagne. The best beverage I put in my mouth on either airline was the Port that Delta served eastbound ATL-MUC. Delta was also serving Bitburger beer westbound MUC-ATL. VS has London Pride in addition to Heineken and Tiger (Singaporean beer brewed by Heineken).

Bar: VS has a three seater bar at the rear of the J cabin, which I happen to enjoy. DL does not have this feature on its planes.

Flight Attendants: We were lucky enough to be served by great crews on both Delta and VS, so this was a wash.

Pre-flight lounges: The LHR Clubhouse is the best airline lounge to which I can get admitted; VS's clubhouses in JFK and IAD are nice, although not as expansive as LHR. DL is renovating the JFK T4 SkyClub, and the Sky Deck is closed to the public at the present time. As much as we like the ATL Concourse F SkyClub with Skydeck (which is open), the bar is basic and the food turns me off.

Post-flight lounge: Landside at LHR T3 there is the "Revivals" lounge which features English breakfasts and hot showers. I was there last month, and I really enjoyed it. There are showers in JFK Skyclubs in both T2 and T4.

Hope this helps.