I only used the Skink and Kugelblitz in BR 3 PVP and PVE matches, so I have not tested them against BR 5 jet aircraft.
TLDR: The Kugelblitz has superior manoeuvrability to the Skink, and is overall superior to the Skink against enemy aircraft, infantry and vehicles. The Skink and Kugelblitz guns will both overheat fairly fast, generally reducing their overall effectiveness. Both the Skink and Kugelblitz are only useful if you play on a map where you can spot and fight the enemy at a safe distance. If the Skink and Kugelblitz are close to enemy infantry they will generally be almost instantly destroyed. Almost all tanks and tank destroyers can destroy the Skink and Kugelblitz with 1 to 2 decent hits. If you play mainly on the USA side then the Skink is the better choice. If you play mainly on the German side then the Kugelblitz is the better choice. Wait 14+ months and you will likely be able to buy both of them at a half price discount.
Long version:
The Skink and Kugelblitz are both useful depending on the map.
Both the Skink and Kugelblitz are effective against enemy infantry, light vehicles and if you know where to hit them certain enemy tanks. You can test both of them out in the practice room against various enemy vehicles.
For the Skink and Kugelblitz to be effective you generally need a map where:
1.) The enemy infantry can be spotted before they are in Dynamite throwing range, so a map where you have a chance to stop them before doing so.
2.) The enemy vehicles cannot spot you from their grey zone and blow you up there the moment you enter the map. See Note I.) below.
Note I.) A few days ago in a match on one of the Tunesia maps an enemy s.I.G. 33 with a 15 cm gun destroyed my Skink in the grey zone within about 5 or so seconds from the moment it entered the map, my Skink joined a number of other destroyed vehicles there which also had not been able to get into the match for the same reason.
If you get a decent map then both the Skink and Kugelblitz can be effective, but you are hindered by their guns overheating far too fast, preventing them from being all round useful.
The downsides of the Skink.
1.) It runs out of ammunition fairly fast. Forcing you to drive back to get ammunition, taking you out of the fight for quite a long time.
2.) It is slightly under-powered (= weak engine) and has limited manoeuvrability. It is fairly slow, even more so in reverse, and it takes time and effort to turn and to get rid of objects blocking its path.
3.) Its 2 cm guns are less effective in destroying aircraft.
4.) In a 1vs1 frontal showdown the Skink will often, but not always, be defeated by the Kugelblitz.
5.) The guns overheat too fast to remain all round useful.
6.) Its armour is fairly weak, it generally cannot stand up to enemy armoured fighting vehicles unless you spot them first and hit them where it prevents them from returning fire.
7.) It has generally weak armour protection. Enemy infantry can fast and easily destroy it with either Dynamite, TNT or hand-held anti-tank weapons (Bazooka, PIAT, Panzerschreck/Ofenrohr, Panzerfaust etc.) and enemy armoured fighting vehicles can usually take it out with one hit. Molotov cocktails too are useful against this vehicle if the enemy knows where to aim.
Tip: Test the Skink out yourself in the practice room against the rotating enemy test target aircraft (= 2 engine bombers). In my experience you cannot shoot all of them down before they fly out of range, about 2 to 3 of them regularly manage to survive. Note that I play with a controller and not with mouse and keyboard.
The downsides of the Kugelblitz.
1.) The guns overheat too fast to remain all round useful.
2.) Its armour is fairly weak, it generally cannot stand up to enemy armoured fighting vehicles unless you spot them first and hit them where it prevents them from returning fire.
3.) It has generally weak armour protection. Enemy infantry can fast and easily destroy it with either Dynamite, TNT or hand-held anti-tank weapons (Bazooka, PIAT, Panzerschreck/Ofenrohr, Panzerfaust etc.) and enemy armoured fighting vehicles can usually take it out with one hit. Molotov cocktails too are useful against this vehicle if the enemy knows where to aim.
Compared to the Skink:
A.) The Kugelblitz ammunition consumption is more economical than that of the Skink, allowing the Kugelblitz to remain in the fight longer in my experience.
B.) The Kugelblitz has better overall manoeuvrability than the Skink.
C.) The 2x 3 cm Kugelblitz guns are overall slightly more effective guns than the 4x 2 cm guns of the Skink.
D.) The Kugelblitz guns are more effective in destroying enemy aircraft than the guns of the Skink, it generally takes the Kugelblitz less ammunition and less hits to destroy an enemy aircraft than the Skink.
Tip: Test the Kugelblitz out yourself in the practice room against the rotating enemy test target aircraft (= 2 engine bombers). In my experience you at times can shoot all of them down before they fly out of range. Note that I play with a controller and not with mouse and keyboard.