Storage solutions aren't cheap. If the price difference is that close, you may just want to buy a 2nd PS4000XV with 8 drives. It gives you some flexibility, capabilities and possibly performance that a single PS unit wouldn't give; different raid types, more iSCSI ports, option to use multiple pools, etc. If you opt for this option you may want to verify that your existing unit is fairly up to date in firmware (i.e. 5.0.2 or later probably) before you try to add the new unit.
I bought one EQloigc PS400XV with 8 SAS discs and 2 controller last year, and now I am planing to buy another 8 SAS dics, but it cost almost the same PS4000XV I bought before. It's beyond my understanding.
Why Dell EQlogic 8 new SAS discs cost almost the same as the new array with 8 
SATA discs?