I am not with Laminar Research; just a flight simmer.
All versions eventually become the same product.
The steam version is a 3rd party product and from earlier comments by Laminar Research staff in this forum there is quite some delay in receiving updates and another comment I have seen by the same LR staff there may be 2 or more parts to the installation. The Steam version does not mix with the Laminar Research version; meaning you cannot update from Laminar Research versions. You have to wait for Steam to have the updates. Personally I don't support Steam.
In relation to the disc version there are two options. Buy direct from Laminar Research or from a licensed reseller for Europe and Oceania (Aerosoft). The Aerosoft version provides additional add-ons and provides a password with the product. The password they provide has no impact on the ability to run the X-Plane software. It is only a ploy to get you to purchase more of their products. The downfall with the disc version is the No 1 disc has to be inserted in the disc drive at all times. Unlike X-Plane 10 there is no USB dongle that substitutes for disc 1. I have the aerosoft version only due to the fact I was able to get a good price downunder considering exchange rates and freight charges fron the USA. The positive aspect with the disc version from either party is you can get any updates, including Beta versions at anytime, when available, when you launch X-Plane ie immediately.
The digital download direct from Laminar Research is the 3rd option. You should, as a backup, keep a copy either on a large USB drive or on another part of your PC (additional drive) away from the official location where you want to run X-Plane. You should also keep in a secure place details of your purchase and password key. There are many situations in this forum where users have lost their passwords. Laminar Research have now setup an email address just to deal with this problem. The Laminar Research digital version will allow you to receive updates as they occur just like the disc version. A possible downfall with the digital version download is your internet speed, and the speed of international servers forwarding/supplying the digital download. There are instances in this forum where the download has taken several hours.
Personally to answer the question you have asked at the end of your question and having tried a digital download myself is to go for this version. Don't purchase immediately but download the demo version. In doing so you will get an indication of the download speed and you will be able to try before you purchase. When downloaded and tried, purchase the product and then apply the password key provided by Laminar Research when you run the demo version. The demo version will then unlock hidden gems allowing you full access to the product.
One of the good things about the X-Plane software when loaded is that, if loaded correctly, it loads as a standalone directory and can be copied/ transferred onto another computer. It does not drift into other files structures eg operating system files. Read the manual first about the installation. The manual can be found at http://x-plane.com/manuals/desktop/
Good luck and welcome to X-Plane.