My opinion may be a little biased because I'm from Valencia, but I think Valencia is the best place for the spectators.
1st of all the Circuit of Valencia has the track lower than the stands, and flat, which means that you can see the whole track from almost every place. It's like a football stadium, but much bigger. I think it's the only circuit in the whole MGP calendar of this type.
2nd, when 100 000 people all try to reach the same place at the same time, traffic chaos is unavoidable, but in Jerez the parking is a disaster, there is only one road to the Circuit which gets completely saturated, if it rains then you have the risk of finding your bike/car semi buried in the mud.
Valencia and Barcelona have better access and larger parking lots. Also Valencia has a train station so you can go from Valencia city centre directly to the circuit. (dunno if barcelona has one)
3rd. The climate, the Jerez and Barcelona are usually very hot because of the time of the year, 30ºC at least. The Valencian GP is on October so the temp is lower around 25ºC during the day and usually sunny.
4th. Its the last race of the year, so riders always go out to the limit because it's their last chance of showing themselves and usually at least 1 championship winner will be decided there.
In terms of party they're pretty much the same, it makes sense because it's the same people that go to the 3 races in Spain !