Reinand is a minimalist code. It is a CFW exclusively for n3ds and has emuNAND and sig patching support. Just recently added support on n3ds for 10.3 emuNAND
CakesFW is fairly similar to Reinand in it's simplistic coding, but supports both n3ds and o3ds, so naturally it is very slightly more bulky than Reinand. Main features is that it also supports enuNAND and sig patching, as well as 10.3 on n3ds. It has a few more features relating to FIRM patching and so on, but if you are a basic user, you wont delve into that as much.
rxTools is the bulkiest of the three, but comes with many useful tools. It supports emuNAND as well, has firmware spoofing built in, sig patching, emuNAND support, xorpad generating, native regionfree, as well as multiple other features. Just go to the main thread for it to see the other utilities. As or this moment, it only supports 9.5 on n3ds, but will soon be supporting 10.3 just as the other two.
Most new users prefer rxTools for it's tools, seeing as how once it is set up, the firmware spoofing means despite the 9.5 limitation, you don't have to get special cia files. Same with regions. All three are just about as easy to setup, so choose the one that you want based on your needs and wants.