I get the feeling you still don't fully understand what RTM is. RTM is also called RTE witch stands for Real Time Memory editing or Real Time Edit. So you can see asking which one is the best doesn't make very much sense.
You can mod games with any of the available custom firmwares. They are generally 1 of 2 types CEX or DEX. If you only want to use tools made by someone else, CEX is more likely your best choice as it operates just like the retail PS3 you have now but with the option to install packages and run homebrew (and backups).
If you intend to make your own mods, DEX is the better choice as it allows connection with the official Sony debugging tools. This connection method is much faster for those that are actually debugging software. With DEX firmware there are many options that are not available on CEX, but more options also complicates things more. Also with DEX you will be required to reset your console ID every time you reboot and want to go online.
There is a 3rd option called REX which is a hybrid between CEX and DEX. It allows easy switching between the 2 modes as both kernels are in the firmware. Probably not what you are looking for to start with.
I suggest you start with a CEX type firmware and if you find it does not do everything you want then go through the CEX to DEX conversion process. Remember to set your QA flag before leaving 3.55 to a higher firmware.