Speed is always first and ASAP. Range is either never upgraded or very late.
For me personally (as anyone who knows me knows I don't necessarily play "correct"), I think the opposite. I firmly believe in range over speed. In pvp it matters a lot less and you can delay the upgrade especially early game because you need gas for reaver, goons, obs, dt, etc etc.
The reason I say sight is better is for a few reasons:
1. I have very low apm. I cannot spend what little apm I have to babysit dumb dragoons AND obs. People will argue that for this reason speed is actually better but in my humble opinion, they are wrong. If you take 11 dragoons and 1 obs and "a move", you will notice the speed obs will run ahead of everything and die to turret, goliath, scourge, hydras+lords, etc etc. If you move short distances (emphasis on short distances, not across the damn map), you will notice that even though your obs won't be ahead, it will have enough "sight" to see any mines and etc.
2. The range is more useful in scouting purposes. Instead of having 2-3 obs in an enemy base, you can just have 1 and see the majority of their base. If you place a few obs around map to see stuff coming, it will help in that regard as well. It also outranges turrets so you will be able to scout the opponent without him even seeing that you have an obs there.
3. In PvZ people argue that the zerg can snipe the obs. That is true. However, to me it's the lesser of 2 evils. I'd prefer to be able to see the lurkers from far away, and send storms and dragoons to try and bust then to keep running the obs back and forth. Since the obs is further back (even though it's slower), it will require the scourge or whatever to come right into contact with your army to try and snipe.
There's a lot more but I'm not high right now so I'm not at 100% mental capacity and I'm also at work. Any questions feel free to message me or reply.