They will be in RAID 0 so it should compensate a little.
The 840 Pro is much more expensive and I don't think they come in larger then 500 GB (the Evo does) which matters as I will use them as my work drive, I have a Samsung SSD as my system drive.
Lastly, my current board only support SATA II and while I plan to upgrade eventually right now it shouldn't be such a big difference in practical terms. As you said, it will be much faster than my current hdd (and silent!).
840 Pro is my current choice if I upgrade the system drive.