I am currently using LEAP 42.3. OpenSUSE has announced the immanent arrival of LEAP 15.
- What is the difference between the two distributions?
- Will 4x.x be phased out?
- Should I switch to 15?
- Does it matter?
The release of Leap 15.0 (the successor to 42.3) will be at the end of May, yes.
- What is the difference between the two distributions?
As always, the latest release will have the latest software.
In this case, as Leap 15 is based on SLE 15 (as opposed to 42.x which was based on SLE 12), this means all packages will be upgraded to newer versions, also those that come from SLE.
Basically, Leap 15.0 is more or less the same as current Tumbleweed, but it is again frozen now as usual.
Think of it as an upgrade as it was in 13.x times, when all releases were completely based on Factory (now called Tumbleweed).
- Will 4x.x be phased out?
Sure, like any other release.
https://en.opensuse.org/Lifetime
It will still be supported for 6 months after the new release, although there were talks about extending the life-time of 42.3 some time ago as 15 is a major new release.
I don’t know what came out of this though, and what the current plans are.
IOW, there hasn’t been any official announcement I know of that Leap 42.3 will be supported for a longer period than the usual 6 months after 15.0.
- Should I switch to 15?
If you want to keep using openSUSE Leap, then yes, at some point at least.
You don’t have to immediately.
- Does it matter?
Sure it does matter, but I’m not sure how to give a concrete answer here… 
The release of Leap 15.0 (the successor to 42.3) will be at the end of May, yes.
Okay, thank you.
Onward with LEAP 15 then!
JFYI, until the final release it will be rolling, so if you install now you can always just further update as usual without changing any repos.
(as was the case with Leap 42.3 already)
Or you can just wait a bit longer of course, as indicated Leap 42.3 will be supported until January 2019 at least…
(there won’t be a 42.4 though, Leap 15.0 is definitely the next version)