Shopify is a standalone payment platform, whereas WooCommerce is a plugin that can be integrated with a WordPress site. 

WooCommerce is free, and much like WordPress it is open source. Shopify has a free trial, but will cost $29.00 a month after that. Both options provide in-depth support either through a community of online forums or in the case of Shopify, contact directly with an expert. 

WooCommerce is used for online transactions, whereas Shopify has online and offline point-of-sale systems. 

Shopify, being a standalone product, handles SSL. For WooCommerce, you will need to find a compatible plugin. Both options offer SEO features.