As a UK based VS Gold, the only benefit I notice is access to DL lounges when I am travelling back to UK and access to SkyPriority lanes.
If you are in the US and doing a lot of domestic and non-London travel, I'd recommend switching to the DL programme.
US domestic upgrades will be close to impossible for VS Golds unless it's a totally empty flight.
In the UK, VS benefits have to be competitive with the big BA. In the US, DL is the big one. Unless your travels are niche - all London - it's better to join one of the domestic airline programmes. I was a UA frequent flyer when I lived there, Economy Plus seats were the main attraction. Of course, all that was before UA's operational meltdowns. If I moved back, and had a choice, I'd pick DL; they run an incredibly reliable operation and the Econ product is quite good.