This article published yesterday reviews Intel's, OCZ's, Samsung's and some other SSDs and gives a lot of useful information like multitasking performance, boot time and so on.
As you can see in the review, X25-M, Vertex and P256 are all fast. I'm using X25-M 160GB and quite satisfied with its performance. You can also read good reviews of the other two drives from members of this forum. According to several sources including the one I linked here, X25-M is marginally faster overall and pretty good at multitasking, but it doesn't seem the difference is big in real life. The sophisticated anti-degradation algorithm and longer warranty of Intel's SSD might be more important than its read/write speed.
As for multitasking, basically you're always supposed to be multitasking because the OS is doing the behind-the-scene work nonstop such as recording onto your SSD which file you opened and when you made changes. This doesn't necessarily mean X25-M is noticeably faster than others in any situation, but if you're planing on multitasking heavily, then a multitasking friendly drive might be a better choice for you. I'm not sure if you'll notice the speed difference, though.
As some other posters said, I think you'll be satisfied with any of the three SSDs. They're all much faster than mechanical HDDs anyway. At the end of the day, it depends on your budget and usage.