Get the 256GB Samsung 840 Pro.
With two 128GB Samsung 840 Pros in RAID-0 you will get higher benchmark Read/Write speeds, but you won't notice too much difference in real-world performance.
There's no fault tolerance with drives in a RAID-0 array, so if any drives in the array dies then you lose all of your data.
When you create a RAID array you never want to mix HDDs and SSDs. The array should be all SSDs or all HDDs. An array's Read/Write performance is based upon the drive in the array with the slowest Read/Write speeds, so if you RAID a SSD and a HDD you're not using the SSD to it's best performance.
I can't afford two 256GB or 128GB ones right now
just one or the other dang school costs too much haha I might just get the 256 now and save up and get another later on and RAID 0 it then.