Consider how you are going to support the longer slider, how long of a slide you will need, weight if you have to carry long distances and if working in tight spaces. I think you will find that 39" is more than enough in most circumstances. Actually, I cut my Igus slider down from 39" to 29" and it works fine.
I am thinking about getting the longest one but wonder if there is any real benefit to the added length?(or other problems like transporting it or rail flex issues?)
The slyder dolly looks like a better design with 8 bearings and many nice optional attachments. Not that much more compared to konova. Thanks for the link