Long story short, I have a bad back. I want to keep it in tip top shape for as long as possible. I think an inversion table will help. Which one do I choose?

Just find something that is well built.  A broken neck would be worse than a bad back!

If you live in a larger city you might be lucky and find one cheap on Craigslist. I found a HealthRider for $100 when I lived in Houston.

I have degenerated disks.  I find that using it too much or at too much of an incline makes me worse rather than better.  

I have found the best for me is core strengthening exercise - dead lifts, mountain climber, the plank, glute bridge, kettle ball swing, swimming, etc.

I have an earlier model of This
Bought it from HSN five or so years ago for around $300. I love it.
Check with your Dr first. It's not a cure all for all back issues.

Teeter Hang-Ups they seem to be the market leader, I had one and it really didn't do much for me and I gave it to a friend. Amazon has a ton of choices.

Teeter.

My back is a mess.....years of lifting and Motorsports conspiring to wreck it. I was staring at surgery until I got the Teeter.

Now, when my back isn't right, I'll hang a couple of times a day until all is well. I swear by it.

i had one for my bad back , it made it worse check with your doc first

I've owned two different inversion tables.  The current one is a Teeter Hangup.  They help me.  Last year I saw a newer model (Teeter?) that had a more intelligently designed style of releasing and locking in your feet.  It had a long handle that did not require a long stretch and effort to engage and disengage/release your feet...less stretching while pulling a lever. Seems  better for your back if you have more serious problems.  I looked for it again this year and didn't see it (Sports Authority and a high-end type health store).  Google will surely turn up a few good options.

eta my current one also folds up/scissors and stands against the wall.