OT: using external RAID systems for datastores?

Hello all,

we are currently running out of disc space of our servers and in addition, we’ve got performance issues during backup at night. Currently we are using two Lenovo TD350 servers with a 1TB RAID1 each. All datastores are on the same RAID1.

Now I need to make a decision:
Option 1:
Buy new internal harddiscs for the two servers (2x 1TB each = 4 new discs) and split the datastores to one RAID1 mounted to / (incl. everything except all running machines) and the new RAID1 mounted to the datastore where our virtual machines are running.

Option 2:
Buy an external RAID housing (to keep an eye on the costs, mabye a RaidSonic two-bay eSata/USB 3.0) and mount them to the same datastore like in option 1 (new partition for running virtual machines).

Overview about the costs:
option 1 is about 1000 EUR for 4x1TB (the 2TB modells will be about 1.500EUR)
option 2 is about 800 EUR for 2x external RAID-box, 4x 2TB Enterprise HDD

What do you think about the option 2? Does anyone using external RAID boxes for virtualization (if yes, what is your modell?)? Do you have any experiences of the performance? Should I only use the external RAID boxes for the image datastore to decrease the IO during backup (using onevm disk-saveas with other scripts around).

a little bit more information:
Most of our virtual machines are providing services, so no user works on them directly (no remote desktops, etc.). Others justs running at night for checking sourcecode, etc.

Thank you very much in advance,