When you build a Raid 0 with 2 Disks with 150 GB. The disks are striped together an you get a cap of 300 GB. The Date will be placed on the stripe.
When you build a Raid 1 with 2 Disks with 150 GB. The Disks are configured that on each Disk go the same data on it. Tha cap is 150 GB.