I'm w/ pigpen on the config but . . .
If you are worried about weight . . . go tubular . . . save 80+ grams in rims and 150+ grams in tires/tubes . . . that is much more significant than dropping 20 grams by reducing your spoke count by 4 spokes. Conversion to tubular drops a half pound of perimeter rotating mass . . . dropping from 32 to 24 spokes only saves 40 grams . . .
28/32 3x brass on drive side rear. I would use 3x nds rear as well however, better bracing angle, tension is closer to the ds. I would also go with an off center spoke bed in the rear if you can get it. Something like an Velocity Aerohead OC if you are going to stick with clinchers . . . this is really important if you are trying to build with ultra low spoke counts. Not sure what is out there for tubulars if anything.
I used to build my wheels with a combination of aerolites up front and round dbl butted spokes in the rear . . . then I tried Sapim CX-Rays . . . I use them and the Sapim Polyax nipples exclusively for all wheels mtn or road. More stable, better tension control, less windup, yadda yadda yadda.
Do your best to keep a consistent hub configuration for your training and your event wheels. Not all cassette carriers have the same dimensions between manufacturers. When you switch from your training wheels to your event wheels, you may need some derailleur adjustment if the hubs are not the same on each of your wheelsets. I prefer to use White Industries H2 hubs for rear road wheels, light, durable, consistent, build a strong wheel, compatible with either Shimano, SRAM or Campy, good bearings, Ti Cassette Carrier body.
Have a good ride . . .