I have 3 of the 4 RTAs you've mentioned: KFL, Billow V2, Bellus
I'll give some pros and cons so you can decide whats right for you.
1. KFL - you're probably only able to do MTL hit on this. You're builds are most likely going to be in the 1 ~ 2ohm range. You can maybe build sub ohm but the RTA just doesn't have the airflow to properly do this. No leakage as long as you bottom fill. Have not tried the top fill method where you block the airflow hole while filling. Some dry hits, but maybe my wick isn't right or I'm just too used to DL and I'm pulling too hard.
2. Billow V2 - You have to build dual coils (no ceramic / silicon blocker). You can probably do MTL (never tried) but you probably wouldn't want to. You can build sub ohm or >1ohm (I've only done sub ohm, 0.2~0.5 range). Great taste. Lots of vapor. NO top fill. I have no leaks or dry hit issues with this tank assuming you wick right and its fairly easy to wick. A relatively "dry" tank (not alot of condensation).
3. Bellus - You can choose single or dual build. You can build sub ohm or >1 ohm (I've only built dual coil sub ohm). Similar flavor / vapor when compared to the Billow (hardly tell any difference, maybe slightly better taste on billow). This RTA is a bit harder to get the wick right. I've had it leak thru the side airflow holes when topfill & not wicked right. Its a "wet" tank, meaning no matter what you do you're bound to get condensation around the airflow holes either when filling or when vaping.
IMO I would go with the billow V2. No top fill but its just an extra step of unscrewing the tank and filling from bottom.
Flavor best to worst is - Billow > Bellus > KFL
Vapor best to worst is - Billow >= Bellus > KFL
IMO its not fair to compare the KFL to either of the 2.
You have another option, check out the newer RTAs out there (the 4 RTA's you mentioned are a bit dated, KFL & 91% dates back to 2013 or 2014, Billow V2 and Bellus dates back to 2015). 2016 RTAs all have bigger build decks (ranging from 13-17mm), velocity style build decks (super freaking easy to build). And then you have the bells and whistles on these newer RTAs such as juice flow control, top airflow control, unique top fill designs, etc.
I personally have the Griffin 22mm, Alleria 23mm, both great RTAs. I have a Gemini 22mm, Billow V3, Aromamizer Supreme RDTA 25mm coming this month altho this has nothing to do with your thread I'm just excited about it