Ive worked with both systems quite a bit, and I can say that they are both very usable. Your choice will depend more on what platform you prefer to work with than anything else. Also, you need to take some things into concideration liek the fallowing:
1. Do you plan on having any dynamic/db dirven content?
if so, you will need to be sure that the web server supports it. For unix, make sure they have php and mysql, for ms, make sure they have asp and either access support or ms-sql server. In general, both platforms are good with this, php tends to run a little faster and is accepted as being a more stable, scaleable platform.
2. Do you have any pre-built content?
If so, this will almost make the dessision for you. If you have alot of pages hanging around with .asp extentions, you will most likely want to go with ms, if you have alot of perl scripts that are for unix, that would be the choice.
Like I said above, both platforms are usable, and both have alot of exposure. I personnaly perfer unix. You will also see that the choices are wider for unix hosting. If you want a recommendation for a host, try
http://www.linuxwebhost.com. They are reliable, easy to use, and to my experiance have not had many if any problems. They support MySQL, PHP, CGI, shopping carts, SSL, PGPemail, mail hosting, etc. For 15bucks a mnth, you get 50 email accounts, 100meg of space, and like 2 gig of band width