If you arn't fussed about PSN access IMO the most stable firmware is 3.55 however signing games for 3.55 may become less common due to the capability of newer cfw(s).
The 'rootkit' you speak off was allegedly that Sony could remotely execute code as soon as you connected to the internet / psn(?) and could check for homebrew etc, i'm fairly sure this would of been disabled if it was true, I believe it was sourced from Mathieulh.
In a stand of between Rebug V4.21 and Rogero v4.XX is less reported bricks, and preferable from a dev point of view other than that both have pre spoofed psn etc, rogero is however more simple to use. I believe rebug is also more stable than Rogero but to each his own, another pro-4.xx point is that most homebrew has been ported over and 3.55 homebrew compatibility is more likely to wither because of the advanced cfws, it may be worth a try and you can always downgrade if you don't like it, make sure to follow a guide to ensure no errors.
0x00, 3.55 in general are more stable with better homebrew compatibility and less glitchy. Quite a few devs support It, Duplex themselves told people to stay on 3.55 as they would continue to release game fixes, so does PropJoe, which is constantly fixing EBOOTs/DLCs.
Rebug REX 3.55.3 CFW have the exacly same feature as the 4.21, but with 3.55 instead. Unless you're crazy for 3.55+ features such as Vita support, I recomend you to stay at 3.55, not only because of the benefits I've already mentioned, but also, because It allows you to fix errors such as RSOD in a simple manner, while If you were on 4.21 you would require hardware flasher.
As for the homebrews, you're right, ps3 scene isn't filled with them as the Wii, but I believe there is quite a few that still aren't working on 3.55+ and perhaps never will. If you only use MultiMan, Showtime and emulators and you're 100% sure you really need those 3.55+ OFW features, then perhaps it's worth getting Rebug's 4.21. If you want to see the updates, feel free to look at Wikipedia.
On 3.55 CFW you can get on FSM or use the NOTHEROS pup fix, on higher FWs you can't and we can't be sure If It will ever be possible, so you'll need a hardware flasher to downgrade back to 3.55 and fix it.