About 3 years ago when I started building space models (I'm really ashamed about what I've done to several nice kits ...) I built the Soyuz/Vostok/Luna kit made by AIRFIX. Now it was time to compare the two kits. The AIRFIX kit includes all needed pieces for Vostok, Soyuz or Luna. You get a Soyuz and a Vostok Spacecraft in 1/144, both very simplified of course. While the 3rd stage + payload for the Vostok version seem to be ok, the pieces for Soyuz are much too simple. Airfix provides the same engine for the Vostok and the Soyuz 3rd stage which is incorrect (correct: Soyuz - 4 nozzles, Vostok - 1). The original 3rd stage is one cylinder - AIRFIX modeled it as 2 cylinder pieces with a cone section between them. That would have been good for the payload section but AIRFIX modeled this as one cylinder. However - if you want to build a Soyuz model it's better to scratch build your own Soyuz upper stage or get one of the conversion sets by Rho Models or RealSpace Models (I'm not sure if they'll work with the AIRFIX kit because they were made for the APEX kit which has about the correct diameter).
The APEX Sputnik model includes the pieces for the first 2 stages plus Sputnik 1 (not Sputnik 2 as shown on the box) as a payload. You also get the adapter between 2nd and 3rd stage which is nice if you want to use it as a base kit for Soyuz or Vostok. The only problem is that you have an "open" 2nd stage. This should be easy to correct.
I haven't started building the APEX kit yet but holding some of the pieces together you can figure out that the part fit is really good. With the AIRFIX kit you will have some problems. Because the two halves of the 4 boosters of the first stage don't fit very well you will destroy most of the fine raised lines if you fill the seams with putty and sand them smooth. APEX has done a better job in this case, not only because the parts fit better, but also because the surface detail is recessed (as it should be - as far as I know) which can be restored easily when destroyed. The original Sputnik rocket had 4 tubes on the second stage leading from the control section to the top of each booster. The AIRFIX kit has them, the APEX kit doesn't. The APEX kit on the other hand has the tunnel for the electric cable (I guess that's what it is) on the 2nd stage. Both can be added easily.
Now for scale correctness. The table shows the length and diameter of the Sputnik rocket as it should be in 1/144 scale compared to the values of the 2 kits.
|1st stage (boosters)|
|2nd stage (without adapter)|
|diameter (upper end)||1.8||1.7||1.8|