Aveline,

I second Arthur's comment

I think that, feature for feature, you will find infusionWP to be the better system for your client.

This is especially true if you want deep integration with Infusionsoft.

Assuming your client has a payment gateway already integrated into Infusionsoft, I would also let Infusionsoft process all the payments.

This would clear the way for your client's site to run on full auto-pilot and it would automatically manage every step of the process, from initial order/signup to failed recurring membership payments.

The basic idea is to use Infusionsoft as the financial "motor" of the system you are going to build. You don't really want to be managing memberships partially through Infusionsoft and partially through Wordpress. It would be a nightmare to keep both sides manually synchronized.

Best,
Bob