Hmm, I also agree that Higurashi should come first rather than Umineko, though like some have said it also depends on whether one will like Higurashi or not. Whilst Higurashi is a really good murder-mystery, Umineko is more complex and it may overshadow Higurashi that has a more humble setting and development, and even if somebody reads Higurashi after Umineko and likes it, I don’t think it would be the same as somebody who would have read it first. Also, Ryukishi07’s storytelling skills develop more and more with each work and his mysteries become more meticuluous, so if you start by reading Higurashi first that is the easiest one to solve out of all the three, you are familiar with his mystery writing technique and are able to understand it better, notice clues more easily and watch his writing develop. And of course, you won’t get much of the Higurashi references in Umineko if you read the latter first and if you want to get into 07th Expansion as a whole you should probably start with Higurashi. Anyhow, like I said, some may dislike it, and Umineko is of course a very complex and well-written story in itself.
The biggest reason I can see for recommending Umineko over Higurashi is this: It presents itself as a fairly ‘classic’ mystery in the first episode, if you ignore one or two instances of questionable anime humour. For someone who doesn’t watch sol anime or read visual novels, a mystery set in a small rural Japanese village that starts out with a group of students with weird hair colours playing games is a lot harder to accept than one set in a western-style mansion, where the majority of the characters are adults and there’s intrigue going on (a good many people seem to find the adults conversations boring, but I think that might be very different for someone coming from a crime fiction background). Higanbana might also be a good candidate for a first recommendation in some cases.
I’d definitely recommend Higurashi first because it is a great VN, but it certainly would be less enjoyable after Umineko. I’d even argue that Umineko spoils Higurashi’s core mystery in a way.
I know a few people though who I think would enjoy both VN’s, but Higurashi’s self-advertisement as horror about killer girls is putting them off and I can’t get them to go past that. Even recommending Umineko to them is meaningless now because of it’s close association with Higurashi. So I think that in a weird way Higurashi alienates some people from the franchise as a whole.