X-Plane every time - the helo flight model is much more accurate than MSFS.
If you do go for MS FS get the latest version - FSX I think, as MS did some work to the helo flight model and improved it quite a bit. Pre FSX, i.e. 2004 & 2002 it's a bit ropey.
On the plus side for FSX it does have more aircraft you can add on, most are available free from various websites - don't forget to download the panels too though.
X-Plane comes with R22, B206 and S61, IIRC. FSX comes with R22 & B206