I think one reason for the lack of responses is the fact that for under $500, you're really not going to find anything that is a step up from SONAR 3.x. SONAR 3 is a fully featured high end MIDI/Audio sequencer. It has all the features you'd expect from a "flagship" MIDI sequencer, although not implemented the way some prefer.
If you want to switch to another fully featured audio + MIDI sequencer in the PC world, you only have two options: Cubase SX, or Nuendo. These applications are more than $500. Although you can get Cubase SX for about $599 or less, so it's not that much more than $500. There is a "light" version of Cubase SX called Cubase SL for under $300 but I doubt you'd consider it an "upgrade" from SONAR.
If you are willing to give up decent MIDI implementation, you can switch to the Digidesign M-Box, which would be exactly $500, and comes with Pro Tools LE. PTLE is world class audio editing, but it's MIDI is very sub-par. Not only a step down from SONAR, but a step down from Cakewalk Home Studio! Of course, PTLE only works with Digidesign hardware, so you're "married" to the M-Box interface...if you don't like it, there's nothing you can do, except buy a more expensive Digidesign interface.
So basically, for under $500, your options are pretty limited. For a bit over $500, you could move to Cubase SX.
But of course...then comes the real question: what do you really need that SONAR doesn't offer you? Don't think I'm trying to sell you on SONAR, by the way. While I review most of Cakewalk's prodcuts for US and UK pro audio magazines, my preference is a Macintosh with Logic Pro. But it's a valid question--what is it that you think you will gain by moving to another application? In the end, that will govern the best option for you.
I've used most of them, and when I first tried Samplitude I was sold. Been using Samp for about three years now and wouldn't ever consider anything else again. Version 8 is just coming out. I do entirely audio only work (i.e. no midi or VSTi) but version 8 is supposed to be vastly improved in these areas. IMHO, nothing can touch Samp