As user of both gateway and CFW.
GW is faster than CFW to boot?
Nope if you use the same entrypoint for both options, both take similar times.
However GW launcher is less reliable, having lower boot rates, and they get worse on some 3DS models (mainly the original 3DS on small factor).
GW is easier to setup?
Depends on what you're taking into account, the most difficult part on both GW and CFWs setup is the entrypoints, that are the same for both.
There are only 3 difficult matters with CFW right now, you must obtain Nintendo copyrighted files by yourself, devs won't host them because they are easier to be found and sued than GW Team. The other difficult is getting the proper CIAs files to use on your setup (Region Free, Cryptofixed, etc), while on GW you generally just drag and drop .3DS files, but if you use the ROM to play online you must inject a header into it, while CIA files doesn't require this extra step.
[EDIT1]The third difficulty (that I've forgot before), is that to use cheats on CFW you must use NTR over the CFW, in GW it's included on their menu.[/EDIT1]
What was hard before actually was to decide what CFW you should use, because when devs started developing CFW for 3DS they had different sets of functions to end user, and it was harder to get a "all in one solution".
Now most of CFW support almost the same functions.
GW is safer to use?
If GW Team removed completely the brick code of their launcher both have the same security degree, if not, CFWs are safer to use.
[EDIT2]Probably people are confusing the security and stability of the downgrading process with the security to actually use CFW and GW.
The majority of the GW user base were already using a 3DS console on 4.X to 9.2 versions, so when the updated sysUpdater was released to support 10.3 they didn't bat a eye, this mattered only to those who had some 3DS units with versions > 9.2 lying and wanted to run GW or CFW on them (like me that had 2 units on this situation).
So yes, it's easy to those who didn't care about downgrading consoles to confuse "I've bricked my console while downgrading" with "I've bricked my console while setting up rxTools/Cakes/ReiNAND", but in reality all bricking was due to errors with older versions of donwgraders or user errors, not CFW's.
And people that updated the sysNAND to 10.4 thinking that they were updating emuNAND would make the exact same error while setting GW emunand.[/EDIT2]