I am thinking about changing the cables on my road bike. I have heard good things about Jagwire Racer, but I have no experience with it. If cost are about the same for the Jagwire Racer and Shimano Dura Ace cablesets, which would you -as a mechanic- choose/prefer?

Thanks for all responses.

Whichever ones are coated in Magical Mountain Climbing Juice.

"Slick" cables are die-drawn. They have a smoother surface than simply cables consisting of smaller twisted wires. I don't think DA cables are die-drawn, so the Racer cables might be smoother and hold less gunk. The Racer's sealed ferrules can create more resistance on the cable. Sealing might be a benefit if you're mountain biking in wet conditions, but regular ferrules are perfectly fine for all other applications.
Sometimes you have to use longer lengths of linear housing to prevent the individual strands from poking through at the ends over time. If the Racer's LEX housing uses a weave, this might eliminate that problem.
If the price is the same, a good compromise would be to use the Racer system with regular, unsealed ferrules.

I have easily noticed that the "slick" die drawn smooth cables are way better than just regular round strand cables. But from there it seems to be mostly mumbo jumbo when you move up to teflon or other coated cables. I've used a couple of such cables in the past and didn't notice any real world improvement in feel over just the smooth drawn regular "slick" cable. However I will say that I've also had noticable improvement in feel when using premium housings. I really liked the old Jagwire kevlar wound housings that were made similar to shifter housing with longitudinal support strands. They really hardened up the brake system feel. No noticable difference on the shifter systems though.

One thing I've noticed with shifter systems is that they do respond well to regular replacements of the housing regardless of which cables you use. The smaller cable seems to cut into the plastic housing lining more easily and when that happens it begins to bind things up. It seems that by the time a cable requires changing on a shifter that the housing needs to be changed as well if you want to restore things to a like new snappy feel to the system.

I've tried three different kinds of coated cables. One version has a black coating that looks like it's just sprayed on. The coating flakes off and wears away especially when it runs under the bottom bracket. Another version, Ripcords, has a thicker, more durable black coating. A third version has a flourescent green coating that's a bit thicker. It too can wear and fray, but it works especially good with simply routing applications like front brakes.
The "roughest" cables I've used are Campy brake cables. They're like guitar's low E string.