Wheelbuilding / Wheel Building / Wheel build help

TB14 to Novatec BDW hubs, 32H 3x comes out as 294.7mm on the rear and 295.3mm on the front. Guessing I should probably go with 294mm for the rear and 296mm for the front?

What's the weight saving/penalty of revs/alpines vs standard DT competitions?

What length nipples should I buy?

294 will be OK on the front too, if you can't get 295.
Weight saving isn't worth having, if you want faster wheels use fewer DT Comps rather than the same number of Revs
Use whatever nipple length you used in your calculations, but 12mm is all you need - longer nips are for rims with a long distance from nipple seat to inner diameter of the rim, usually V-shaped ones.