Do you have to level your cakes? I was thinking if I did a 2 inch pan and level it then I would have less than 2 inches and a shorty cake. If I did a 3 inch pan then I would level it and have a 2 inch cake. I have a 2 inch round and I hate it cause once I level it there is hardly any cake left.
Also do you do 2 tiers with your sheet cakes like you would for a round cake or do you just tort?
I always use 2 inch pans. I bake such that the cake rises slightly above the edges so that the cake is the height of the pan after leveling. I only do a 2 tiered sheet cake if they need that much cake, usually it is a single layer!
I always tell my customers that a 9x13 sheet cake serves about 29 people. That is for pieces 2x2x2 inches in size (ie. a single layer cake). If it were a double layer then it would serve about 58 people for 1x2x4 pieces. Those are the measurements I go by!! 
I think it depends on what type of cake your baking too. It takes forever to bake a 3" cake, and mine usually dont get done all the way through (even with the flower nail in the middle). If your cakes bake up nice then maybe a 3" is ok.
I always tell my customers that a 9x13 sheet cake serves about 29 people.
29 is a prime number so I can't figure how to cut a cake to get 29 pieces.
when cut in 2x2x2, the 9x13 would be cut in approx 4 rows by 6 columns = 24 servings.
I would also go with the 2" pan since sheet cakes are traditionally 2" tall, single layers.
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