I've a big problem with amp sims in that I own real, hardware versions of much of the stuff I would need to simulate to get my basic setup.
I've yet to find an amp sim that actually sounds or behaves like the hardware version of what they claim to emulate. They also require very careful gain staging unless you want your low-output vintage style Strat or Tele to sound like some pointy heavy metal machine. The Sansamp Character series are reasonably close and, in my opinion, react and feel much more like and amplifier.
Out of the software emulators I find Amplitube the least offensive, though its models of the Orange Tiny Terror and Fender valve reverb don't sound or feel like the real thing. Nor for that matter does their Marshall plexi.
Another option is something like the Blackheart B1H1, a quarter watt valve amp that's not outrageously expensive. Add a small speaker cab and it's a very nice setup at domestic volume levels.
One thing to watch with vst amp sims is latency. You will be monitoring the guitar via Sonar's input echo. Unless your DAW and interface can give you a "round trip" time of 10 milliseconds or less expect to find it rather difficult to play accurately (or at all) as the gap between playing the not and actually hearing it come out of your monitors gets very noticable once latency gets much over 10ms.