With 2 drives, RAID 0 would be your best and pretty much only option, considering your aim of keeping disk space.
However RAID 0 reliability is roughly estimated as Mean_Time_To_Failure / Number_Disks, which means on your case, half the MTTF of your 128Gb SSD. Not good.
Furthermore, while you may get double the performance on a RAID 0 scenario under optimal conditions, this will only happen rarely on reads and writes smaller than the stripe size and with evenly distributed blocks. Depending on what you do with your machine you would have to play around with the stripe size between (32Kb, 64Kb, 128Kb, 256Kb or even 512Kb) to get the best of your RAID. Too much may seem good, but will mean less evenly distributed blocks. Too little and you will only rarely benefit from increased performance.
Get the one big disk. SSDs are already a very good improvement over platter hard drives:
- You get a guaranteed 5-10% performance boost with no questions asked
- One disk is much simpler to administer than a RAID
- Your disk reliability is increased over a RAID 0