First to the tagging support. Since I started using del.icio.us a few months ago I came to love this kind of simple but efficient keywording of content. It helped me quite a great deal when it comes to find things much faster so I'm now at a point where I start tagging as much as I can for faster access. Both services support tagging of links, so none of them has really an advantage here.
Also publishing bookmarks is possible in del.icio.us (implicit) and spurl (explicit). In del.icio.us other people can view your bookmarks more or less like you see them yourself which gives the whole system some really strong "community" or better "open" feeling. In spurl you have to explicitly publish your bookmarks. They are (as in del.icio.us) accessible through a "recent additions" page but from this page you don't get to the bookmark collections of other users since only the number of so called "spurls" are listed there for each bookmark and not also the names of the people having spurled this link. In this category I prefer the del.icio.us approach but combined with a way to make some links not public (for example with a reserved tag). Something like this can be done by protecting a bookmark in spurl.
As I've already mentioned: In del.icio.us you see other people's bookmark collections more or less like you'd see your own. By clicking on one of the tags assigned with any bookmark you see on the del.icio.us you get a listing of bookmarks tagged with the same word. Spurl has a slightly different approach: You don't see what other people have tagged their links with and you also can't explicitly only search for tags. Instead Spurl uses a search engine to search through all the bookmarks in your collection or in the whole system using the title, tags and descriptions as well as the full-text of the content. Since spurl also creates cached copies of bookmarks pointing a document not larger than 256kB this is quite a great extension of this caching. For this feature I'd again love to see a merge between these two services: A tag based browsing (also of other people's archives) with the option to also use a search engine for full-text searching.
For me probably the strongest point is, that the service has to stay free. Spurl states this publically on the about page while I couldn't find such a notice anywhere on del.icio.us. Being a student I don't have much money at my disposal which most of the time gets spent on some CDs, DVDs and videogames. Another not so selfish point is, that social bookmarking services live through the community. Without users that would have no content. This is also part of what I don't like about spurl: There only seems to be quite a small community yet (small but good ;-) ) and there is no easy way to access other users' bookmark collection since nowhere is the name of the users listed. There could also be some kind of midway here: Furl AFAIK lets people write reviews or public comments on links which then appear on the "details" page of the link. This way Spurl could add some optional community around certain links. Optional because only these public comments would contain the name of the author and assoziate them with the link.
Another small thing I noticed: I somehow can't join the del.icio.us mailing list and also contacting Joshua Schachter directly somehow seems to have failed. In this case getting in contact with the team behind Spurl is much easier ;-)
For me personally the current state of these two projects bears a small problem: I can't really decide which one to use exclusively. Using both long-term is quite inefficient and takes more time then I want to spent on it, so I will have to decide which one to use. This is not a decition I will make now or tomorrow, but it will have to come.