v5.6 vs v8
1) 5.8 supports PHP 7 out of the box, 5.6 does not, but can be modified
2) 5.8 supports layouts based on framework columns, 5.6 does not
3) 5.6 has less bugs and there are some features you can find in 5.6 that are not yet into 5.8
The upgrade process will not be easy, there is no direct upgrade so you will have to more or less rebuild the site with 5.8 and then use the migration tool (https://www.concrete5.org/community/forums/customizing_c5/5.6-to-5.7-migration-now-available) to move your content. Before you use the migration tool you'll have to re-create all of the custom pieces on your site, you can not install a 5.6 package on 5.7+ sites without quite a bit of updating. Also, any add-ons you have purchased will have to be re-purchased for the new version and might not have been updated yet.
For small sites this isn't that hard and the improvements are worth it. But for a project of that size, you're much better off sticking with 5.6. It's a fine, stable product and still gets bug fixes and improvements here and there.
As well as the edit and dashboard interface differences, the whole 5.7 and v8 programming environment is different (and is still evolving). For most part-time developers, v8 is a lot harder to develop code for. For a really scary example, look in the v8 docs about styling express forms and compare that to providing a simple alternate view template for a form block in 5.6.
- Do I need the new features in version 8?
- Do I want to re-code all the things needed, since it's a whole different code base and lots of code will not work as with the existing 5.6 site?
- What is my budget (be it the hours you want to code yourself or the amount of $$ you'd like to spend)?
- Will the site be live for another few years (even after the upgrade)?
- Am I scared my site will not work in the near future (as said, 5.6 will not work out of the box with PHP7 and higher, and most hosting will update automatically and therefore cause your site to crash since the day this PHP version is served)?
Version 8 is a complete overhaul and structure (and code) wise a very good improvement (meets the current coding standards). 5.6 is outdated by now but some just prefer the way it works over the current version(s).
Once everything has been done to make it work on PHP7, there's no way to know when a new PHP version is coming out that will yet again make things go "Kaboom!" so-to-speak. It's a technological world after all and one can't forsee the future.
Security issues will be released as patches they said, but I don't know if they will (ever) make an official release to support PHP7 (and higher).
If its on a private server, the whole environment is under your control, so you don't need to be concerned with a shared host platform imposing php7 on a you.
Here's a pretty thoughtful list of features and reasons to use concrete5 and not concrete5 legacy: