Basically none. The difference would be if you went with a deep (40+mm) rim. Reducing spoke count reduces weight, and with improvements in technology, manufacturers can still produce a stiff and solid wheelset. Granted, if you break a spoke with most of the "boutique" wheelsets as some people refer to them as, chances are you won't be riding home.
I crashed last summer and was running a set of Bontrager Race Lite wheels that came stock on my Trek 5200....a paired-spoke wheelset. I hit a rock in the road at around 23 mph, hit it hard enough that it bent the crap out of my wheel (but didn't break a spoke...only bent it). Ride over. Now...would that have happened with a conventional 32 spoke wheel, I dunno....and I don't feel like trying it again to find out.
I just took delivery of a set of Niobium 30 wheels from Mike Garcia (the man, the myth, the legend), and had it built up with DT Super Comp spokes, 20f/24r spoke count, radial lacing up front with 2x in the back, brass nipples on the drive side and aluminum nipples on the rest....Speedcific Serenity hubs front and rear, and his standard quick release skewers. I had these built specifically for racing; wanted a set of bombproof wheels that were slightly more "aero" than my Race Lites, but also something stiffer. Given the fact that I'm under 150 pounds, I can get away with the lower spoke counts. I then decided to go with the standard steel skewers rather than the Ti ones because Mike said the steel ones are much stiffer than the Ti ones....but nearly twice the weight. The wheelset, including rim tape and skewers, is around 1570 grams, so I'm pretty happy with that.
So....I just paid under $500 for a decent little set of wheels. Will they be as "fast" as say a pair of 58mm Zipp carbon tubulars? No. But, I spent $1,500 less and won't worry as much about trashing a rim.