I would say that the answer to your question is It depends...
It depends on a number of things:
- What are you selling?
- Who are you targeting?
- Are visitors likely to show up with a budget in mind?
- Are visitors likely to just purchase one thing and leave?
Amazon.com is highly successful and they only show the total quantity count of the cart. This works very well for me personally because if I'm putting it in my cart then I simply need it and have settled with my rational for paying the price displayed when I added it to the cart.
On the other end of the spectrum I absolutely love shopping on Newegg.com and their shopping cart provides a running total which is excellent when I am pricing out a custom PC because 10 out of 10 time I have a budget in mind. Newegg allows me to do "window shopping" and add things to my cart at-will and once I see that I have reached/exceeded my budget then I evaluate my cart and swap/remove items until I am in range of my budget. I do not price out PCs on Amazon.
Now to answer your direct questions:
Which is most intuitive for the user?
See explanation above.
Which is better performing for the host
Don't worry about it unless you are using an inept programmer. If you are successful and selling a lot of products then won't you buy better hosting?