I've used AutoCAD for work for the last 15 years or so, so that's my preference when it comes down to making patterns. I still use a VERY old copy of R14, which you should be able to pick up on ebay or the like for less then $100 (I've seen 2008 on there for about that).
I'm also quite fluent in PhotosShop, but less so in Illustrator so I'm not sure how they compare. The advantage of a CAD program over a photo editing program is that in CAD the lines are "alive", and you can interact them with one another. As such, if you want to clip one line where it intersects another, that's pretty easy. Likwise, if you want to extend a line to meet another, meet one perpendicular to another, or blend a curve through a series of points, all are straightforward. In Photoshop you can use layers and overlay things, but the lines don't really interact. It's also very easy to identify the areas where backgrounding is required, because you can get the program to fill those spots with color.
Where I love to integrate Photoshop is in distorting and stretching images, which is does like a champ. Oftentimes I'll start with a base image, manipulate it as I like in Photoshop, then import and trace it in CAD. The lines are then live in CAD, and I can proceed as I'd like from there.
Scanner-wise, I have to admit that I cheat in that I have easy access to the photocopier at work which they don't mind me using. I'll use the scanning bed to scan large objects in sections, then stitch the images back together in photoshop. If I didn't have that, I'd go out an buy a bed scanner that would handle 11x17s (which are becoming harder to find).
Likewise, I print everything on 11x17s, though I just do that at home on an ancient color inkjet. I can them tile them back together with a strip of tape back and front, and I use those directly on the leather. They don't survive more then a single project, but it's easy to just print another rather then having patterns around the shop.
The learning curve is steep with CAD, but with a good book it's not TOO bad if you're a little computer savvy. I'm self-taught, for example, and it created a skill set that I then use at the office too.
I'll attach an example of a pattern I'm developing as my first attempt at a saddle fender, which hopefully is somewhat along the right lines (my first effort from scratch). I scan everything I like, and this one started with the Hape saddle fender shown in a Sheridan carving book along with a set of flower and element patterns included with one of Chan Geer's DVDs.
Even though the saddle fender in the book isn't a scan and so everything is a little distorted due to perspective, it provided a nice means of learning how to flow one grouping into another. Then I arranged the groupings with the proper spacings, and arranged the flowers with their stems blending in properly. I then added in flow lines (red), and adjusted the groupings accordingly (I don't know if it's verboten to use circles of different sizes, but I quite like the effect). Next the leaves got added, and I'm starting to blend their stems into the vines. I've slowly started removing the overlapping lines, and filling in areas of backgrounding in yellow. Blending the vines together seems like it would be easier to do by handso I've printed this out and will do that in the next few days, and then I'll re-scan it and digitize those transitions. Last will be to print a final copy to use as a tracing pattern, and a second as a cheat sheet in where the backgrounding will go (I find that after 5 hours of carving my bleary eyes and tired arms appreciate a cheat sheet there).
Anyways, hope that helps,