I have used a few free ecommerce plugins but I have a client who will be having an extensive ecommerce need I’d like to know why or if I should choose shopify instead of woocommerce. Is there anything it offers that woocommerce with addidtional plugins does not offer?
Before you do anything I’d recommend getting a complete specification as for what your client needs for their products, and business needs. Once you have the details then you can look for a cart system that best batches their requirements.
There are 100’s of different shopping cart platforms available so there’s no need to stick with just the popular ones 
What kinds of specifications should I need to know about? Or what kind of questions need to be asked to get these answers?
As far as in aware they will be selling lots of different items and services and offering dropshipping servicewithin the uk
I much prefer using well known services, that way you have know how about reliability and support
Well for a start:
Do you need to cater for products that have complicated options or variations?
Do you need advanced search/filter capabilities or is general category browsing + general search enough?
Do you need to offer grouped products for sale as one product?
Do you need to offer things like “related products”, Bestsellers, Special offers etc?
Do you need customer accounts? If so do you need the ability to offer different pricing or browsing options for different member groups?
Do you need to offer automatic discounts, eg buy 2 get 1 free, or category based discounts?
Do you need to be able to offer different shipping options based on the ordered products or the customers location?
Do you need to cater for different tax rates in various countries /localities?
How easy is it to adapt and modify store templates?
How easy is it to extend the functionality of the system should your client ask for something special?
Does the cart system support your chosen payment gateways?
Is the cart system flexible enough to accommodate presentational changes you may wish to do, eg using different layout for different category views
Will the cart system be easy enough to understand and use by your client? (this one can make or break the project, if the client can’t use the admin it will reflect on you)
Does the backend order system provide the right facilities you need, eg order statuses, emails, flexible email templates
What is the total cost of ownership?
As you indicate you want to use dropshipping does the cart support notifying your suppliers of orders that they need to supply?
…that’s just a few things off the top of my head. It’s well worth talking to your client and going through everything, you don’t want to be landed with a system that’s only half up to the task. Plan wisely first and it will make your life much easier!
o, thanks for the pointers here, will look more into it
Could you be more specific on the “extensive ecommerce need”? It would help to answer your question.
Woocomemrce does seem to be the solution for most WordPress-based stores, but if your client has thousands of items, you might even want to consider going to a CMS that is designed just for ecommerce like Open Commerce and Presta Shop or even Zen Cart.
Can you please explain your eCommerce need?
There is no shortage of plugins to transform your WordPress installation into a web store. You can get exclusive range of Ecommerce services from Inventory Management to the installation and customisation of Ecommerce carts, including Magento, WP-Commerce, Prestashop, Zend Cart and CS Cart.
Recent experiences with Shopify and then trying to find a hosted alternative has taught me the hard-lesson to go with a self-hosted solution from now on.
So I’d say Woocommerce would be better than Shopify, though have a look at OpenCart, I used it in the past and it was great.