With the cushion in place the sides will be about 12" above the cushion. A comfortable height, though perhaps a bit high for some people. In actuality, these 'sides' were never intended to be used primarily as armrests. They can be used with a small pillow as back support should one like to cozy up with a good book (like one would do on a window seat) but the primary function is to offer 'shelter'.Your rendering looks a lot lighter and I like it better. In order to get a better idea, maybe you should include in the render the cushion, because as it is now the sides look too high to rest your arms on them if that is their intended use.
Armrests are tricky to get at the proper height, some people like them a little higher than the elbow once seated, and some prefer them almost under the armpit. A compromise height in the middle of your ribs ( about nipple height)tends to be the best