Clematis armandii and clematis terniflora (aka sweet autumn clematis or SAC and formerly known as clematis maximowicziana and incorrectly as clematis paniculata) are not one and the same. They are two drastically different clematis that are generally as you described. Not sure what nurseries you are seeing them listed as one and the same plant in, however I would tend to shy away from them as sources if they don't know the difference between the two.

One other thing is that sweet autumn clematis and clematis texensis are also two different clematis. In fact, clematis texensis is a native species clematis from Texas with reddish blooms that blooms midsummer and clematis terniflora is a late summer/early fall blooming species clematis from China/Japan that has white blooms.

Realize that C. armandii may not be totally evergreen here in NC depending on where you live. In the Kernersville area, I had one at a previous house which would often drop quite a few of its leaves in bad winters and if planted where it received a lot of northerly winds, the leaves could scorch and drop off. This clematis can often be found at farmer's market and at some of the major big box stores such as Lowes or Home Depot. Chalk Hill is no longer in business as retail seller of clematis. Dan at Brushwood Nursery normally carries it but he is out of it now. He might have it back in stock in the spring. Joy Creek has it but shipping might add prohibitively to the cost of the plant unless you have other things to order from them. I would check around your local big box stores and farmers markets and see if they have one or see if Dan at Brushwood Nursery is planning on having it available in the spring.

Here is a link that might be useful: Armandii at Brushwood

Thank you so much. Several nurseries told me they are the same. I was going by the bloom time as much as anything. Glad to know the "scoop".