Hi Walker,
I've flown United First Suite it is an excellent product - but it's hard to compare with UCS, personally i would always go with VS if it was a choice between the two but i'm VS biased anyway.
The two products are really different United's suite is a full first class product with a first class price to match if money is no object go with united more cabin space window seats are good if traveling alone they are like BA first where the seat is angled towards the window, only 10-14 other seats in the cabin depending which aircraft your on but it's heavy on the wallet about £6500 LHR-JFK.
All depends what you want from your flight if you want privacy do some work, get some sleep go for United, with me i like being sociable on a flight sitting at the bar, chatting to the FA's, the up beat, fun, fresh image of flying - then go for VS. If your talking about the LGW-MCO route United don't fly direct you'll have to go through IAD, and on VS you won't get the bar or the in-flight beauty but can go direct.