Where to put a webstore?

[avinash]

Well it totally depends on you and your site.

Sub-domains are considered as separate domains you will have to build a separate link profile in order to get your products ranked.

Exactly my thoughts.

If you are already ranking then it will be a booster to your conversions. If you can get conversions just by adding your product then why bother to do SEO all over from scratch for a different sub-domain.