Which Shopping Cart to use?

I would like to move my company’s website to WordPress. We need some sort of shopping cart system, but there are some specific features I need the site to have, and I can’t find one shopping cart system that has the features I need. Specifically,

a) Products where each variation can have its own image. For example, if I have a t-shirt shop, with multiple colors of t-shirts, I want the product picture to change based on the color selected. Ideally, I’d like to be able to have several options, and the product image will change based on all the options selected. Showing the customer the exact item they are ordering.

b) Products can be compared. I’d like each product page to have a “check to compare” checkbox, so that as the customer is surfing around the site, he can select multiple items to compare, and then once he’s done, he can see a comparison chart.

c) Customer “groups” that get their own pricing, or at least a way to offer special pricing to certain groups.

I know that many shopping carts have some of these features, but not all. The closest I’ve come to is with Magento, but it’s so huge and clunky, and seems to take up way too many resources. It seems like overkill for what I need.

If anyone has any suggestions, I’d be grateful. I have looked at other CMS options like Joomla, but I like and understand WordPress better. I know I can do this with WordPress, but the question is how.

Thanks so much for your time!

AJ