Don't count out Data's non-organic factor. A number of times he was the only one not affected by some weird thing or another which messed with all of the biological members of the crew. Granted you could respond that he is affected by things that organic crew members aren't but he's the odd man out in that case. Some weird psychedelic effect comes along that affects the crew? Data is immune leaving you an immensely capable officer to try and clean stuff up. Some weird technological effect comes around which affects Data? You still have the rest of your crew. If Spock was your first officer you'd be left with a whole crew suffering from whatever it is in the first case. Oh, also, Data isn't going to get ultra horny every few years and mess everything up..
I'd pick Data. Spock's organic nature makes him the same liability the rest of the crew is, his mind powers could be replaced by other Vulcans or Batazoids or something, and while he may have a bit more of the intuitive human factor going for him, he's just not as "smart" as Data. On top of that, he can directly interface with technology when necessary.