ADR class 6 and class 2 have the highest payout for some types of cargo (ex: acid, which has a price coefficient of 2.20). However, because ADR deliveries only have a single level, they're not the way to make more money. Heavy cargo has (in general) a higher price coefficient than fragile cargo. Since you can increase your % bonus by leveling these up, with Heavy Cargo at level 6 and just-in-time at level 6 (since these combine), you will get the most money going with those skills.
That being said, Fragile cargo has a slightly higher XP bonus than Heavy Cargo (22% vs 18%). So if you're looking to squeeze out more XP rather than money, then you should go Fragile cargo level 6 and Just-in-time level 6. Personally I feel that leveling up is fast enough already, and you'd be better off going for the cash option of Heavy Cargo + JIT.
You'll also want to level up Long Distance, since this will give you reward bonuses that stack with the other skills.
A typical game would look like this for me: Heavy, JIT, Heavy, JIT, Heavy, Long Distance, Heavy, Heavy, Long Distance, Heavy, JIT, JIT, JIT, JIT, Long Distance (up to max). Lastly I'd spend on eco-driving.
Two final tips: When searching for a job, always list by "Price per distance" in order to get the maximum payout per mile. Learn how to park the trailers and never skip parking them manually, so you always get the xp bonus for that.
ETS 2 has a file contains info about levels. Here is fragment of that file:
# Level names "level_limit|level name"
#
level_names[]: "0|@@lvl_name0@@|0xFFe0e0e0"
level_names[]: "5|@@lvl_name1@@|0xFF45c294"
level_names[]: "10|@@lvl_name2@@|0xFF75baea"
level_names[]: "15|@@lvl_name3@@|0xFF3a88f4"
level_names[]: "20|@@lvl_name4@@|0xFF5847f0"
level_names[]: "25|@@lvl_name5@@|0xFFda9356"
level_names[]: "30|@@lvl_name6@@|0xFFf58493"
level_names[]: "40|@@lvl_name7@@|0xFFc99ef2"
level_names[]: "50|@@lvl_name8@@|0xFFc2f9ff"
#
# experience points to the next level
#
level_xp[0]: 200
level_xp[1]: 500
level_xp[2]: 700
level_xp[3]: 900
level_xp[4]: 1000
level_xp[5]: 1100
It is easy to conclude that after level 50 your name of rank will stay the same. And the maximum level, according this long file, is 150 (but I can't say for sure if any new patch changed that, it's confirmed after patch 1.2.5).
Best Answer
It improves aerodynamics when pulling high (tall) cargo's, but I'm not sure if the game would simulate that, as there are numerous useless, just for visual enjoyment upgrades.
It doesn't unlock anything and doesn't have any additional features, so... yeah...