I assume you are writting HTML ?

In which case, the choice doesn't matter. Linux and Windows are just 2 different operating systems used on servers. They have no effect on how you write your site.

Linux is stable and reliable, Windows tends to cost more. Many Host's will give you the choice of either.

Because yours is an e-commerce site, you'll need to consider which script you'll be using for that part of the site.

If it's written in ASP, you'll need Windows hosting. If it's PHP you'll need Linux.

oscommerce is PHP, so you'd need a Linux hosting package.

Given the choice, I'd go for Linux every time.

All the best - Duncan