Please explain the differences in opinion between the Intek radios you described.
It is too early to have opinion about all radios. The 5050 have 3 days, the 3030 received today. I can say something more about 2020 and 4040.
Both (also 3030) have identical casing. Difference is only in lenght of antenna (4040 has longer and straight, 2020 have short spiral). Also opened the casing of both radios. You can see difference:
Left is MT-2020, right is MT-4040.
All is
almost the same. There are smal differences in descriptions on ceramics elements (cristals and filters)- maybe some are better? Also PCB are dedicated for both types. On first look are identical, but 4040 have string "MT-4040" on copper - it also means the differences can be.
Size and weight of 2020, 3030 and 4040 are same (only other antennas). Weight (with batteries) is about 140 g.
The 5050 is much bigger (see picture on my forum), weight is 260 g (because have more and bigger batteries).
Sensitivity
I tried to measure distance of work with vario. Both radios worked good. I live in a city so it is hard to find free space to compare. Once a time going to the office taked both radios with mem, leaving working variometer at home. On dostance below have clear signal for both:
When i hide against building both radios start loosing the signal in the same way - i can say: both have identical sensitivity. I can espect the 3030 will have the same. Completly don't know why 4040 cost 2 times more then 2020 or 3030

The 5050 on first look is the new construction with bit other software. Not compared it yet. Will do it soon.
On first look is much bigger, haevier, and have potentiometer for sound volume.
Also compared sensitivity of 4040 with other, cheap Intek radios like MiniCom Duo, or SL-01D. Is noticeable bigger for about 30-50% (it works on longer distance - means is more sensitive).
Accesories
All radios have included wall charger (looks identical) and NiMh batteries (packeted 4xAAA for 2020, 3030 and 4040, also can work with separate AAA cells). The 5050 have 5x NiMH AA cells.
The cheaper radios: 2020 and 3030 have simple earphone and microfone. The earphone myst be plugged inside the ear, so is not comfortable.
The 2020 and 3030 are sold in pairs, the 4040 and 5050 as simple device.
Any more question?