How to set up an optimal scratch disk?

Hey guys. Having some trouble trying to research this online, so I figured I may as well ask here.

I want to set up the best possible scratch disk solution for photo-editing. Now, here's the thing. I use Lightroom and Photoshop. With Lightroom, I keep all my photos organized into catalogs, and the catalogs are all on an external drive - I always have the drive with me when editing. However, I also use the same drive as my scratch disk for Photoshop.

Now this current set-up makes me a little nervous, since I've heard that using a drive as a scratch disk wears it down much, much faster. However, I don't want to use 2 different drives - one as my catalog drive for Lr, and the other as a scratch drive for Ps. Not very efficient nor portable.

Currently, this is all set up to work with my older MBP through firewire 800. Just ordered a brand new MacBook Air, maxed out. Will finally get to try out USB 3.0 for a change. With it, I want a better scratch disk solution that is affordable.

Right now, I'm thinking I should get a USB 3.0 enclosure with a 7200rpm drive, 500gb or more. Then break it into two partitions, one for my catalogs, the other for scratch disk. Is that a good idea? Will using one partition as a scratch disk put more wear on the drive as a whole?

Or would an SSD be a better option? After all, then I won't have to worry about wear and tear on the drive as much. But is 7200 fast enough that I won't notice the difference, or is USB 3.0 still the bottleneck when it comes to read/write speeds between a 7200hdd and an SSD?

Or, should I get a smaller SSD exclusively for scratch disk, and a separate drive for cataloging? As annoying as it would be to have to use 2 drives all the time, if it's the best solution, I may go with it.

Thanks for your time!