When you say backpack model do you mean a folding wheel?
I own a lendrum original, which is a folding wheel, a little larger than some folding wheels but still a space saver and easier than a 'non-folding' to cart around. I didn't buy it because of the folding feature though, I bought it because I tried the other wheels and that was the one that, to me, felt most comfortable and spun the smoothest.
Sometimes the folding wheels are a little smaller than other wheels, and this can effect how comfortable you are at a wheel (some like the smaller size, and some don't, it's all personal preference).
Choosing a wheel is a very personal decision and I'd really suggest that you see if there's a spinning store or spinning guild within driving distance that would be willing to let you try a few different kinds of wheels before you decide on one. You may find out your favorite wheel is a folding wheel!
The pros and cons to a folding, or castle style, or any style really, can change from brand to brand. For example the Lendrum Folding wheel spins like any regular spinning wheel, but the Magicraft 'Little Gem' (a small travel wheel) has some sort of a drive band acceleration to compensate for the small wheel - I found that it felt a little odd when trying to use it, but others really like it.