Is OS on 120 SSD faster than it would be on a raid0 on 2 sata3 drives?

Yes.

OK, some clarification... it will boot faster, it will launch apps faster. If it's a modern SSD and is one of the faster ones, vertex, 830, m4, etc, it will transfer data faster.

A SSD has far superior random write/read performance than two RAID0 HDDs. Since most OSes, including Win7, contain large amount of files in a variety of sizes, high random read performance would grant the fastest boot time.

However, the two HDDs are more cost efficient in sequential write/read performance, such as video recording.

Unless if Win7 consist of only a few gigantic files, the SSD would give better boot performance.

YES

  Thx for the quick & informational answer. I should have included one other point, Gaming (COD4). this is also faster as well correct? Thanks in advance. USMale  

Best answer selected by theusmale2.

I don't know how you missed the memo but a quick search of this site or any part of the internet will tell you that there is no scenario in which a HDD (or many HDDs) will out perform a SSD.

Ok there may be a couple scenarios but they are exceptions to the rule and your case is not an exception.