The Superhero Database Classification number, or SHDB Class, is a number that represents the overall 'power' of a character. All traits of a character are used for calculating the Classification.
What it DOESN'T mean
This doesn't mean that a higher class would always beat a lower class character. But the bigger the difference in Class is, the more obvious it is who'll win in a fight.
How is this calculated
( INT^1.3 + (STR*0.5 )^2 + (SPE*0.5)^2 + DUR^1.6 + (POW + (SPS*SPL))^2 + COM^1.8 ) ^ TIER
Super Power Score and Level
Every Super Power has a score (SPS) that is used to calculate the Class. Each Super Power also has 3 levels (SPL). The level is set when connecting that Super Power to a character. The level determines the final score, of the Super Power, being used in the calculation.
The main arguement for Venom is that he is more of a mid-close range character, compared to Hulk's get-up-in-your-face playstyle. Plus Venom also has the extended mobility option of grappling forward, However, Hulk's jumping option kind of counteracts that (to some extent, it still slightly makes diving attacks less probable). Plus, when Hulk dies, if you aren't being ganged up on, you should be able to survive a little bit as Banner, and that gun absolutely SHREDS for some reason, so I often gets kills as Banner.
Also Hulk is just a bit easier to play too.