This thread will go over the basic differences in JTAG and RGH consoles. While this may seem somewhat redundant, some people still don’t know the difference. 
NOTE: This thread is not meant to be a tutorial for either modification, nor is it meant to detail every little difference. This is just a basic over view of each modification.
Let’s start with a basic overview of what each of these modifications do, shall we? 
A JTAG/RGH exploited console is modified to enable the running unsigned code. Both of these modification methods achieve the same thing, just in different ways, and they both have different requirements.
The JTAG modification is a software hack based upon an exploitable SMC which is why it can only be done to Kernals 7371 or lower and was patched in the 8XXXX+ kernals.
The RGH modification if a time based attack on the CPU that glitches the chip, and allows a moment for the exploit to take place and unsigned code to be run.
You should start by knowing the following: what Dashboard you are on, what type of 360 you have (Phat or Slim), and what motherboard you have (See Spoiler Below)
[details=Open Me]Phat:
-Xenon (2005)
-Zephyr (2007)
-Falcon (2007)
-Opus (2008 )
-Jasper (2008 )
Slim:
-Trinity (2010)
-Corona (2011) [/details]
To JTAG your console, you’ll need to meet the following requirements before you tear apart your 360 and cry because you can’t JTAG it:
[details=Open Me]
- Dashboard lower than 7379 (And even then it’s iffy)
- The correct CB version (Circuit Board version, can only be found by dumping the NAND)
Slim 360’s CANNOT by JTAG’d, but can be RGH’d (Given it meets the requirements)[/details]
To RGH (Reset Glitch Hack) your 360, you’ll need to meet the following requirements:
[details=Open Me]
- Dash 14719 or lower
- The correct CB (CB’s changed during the 15XXX update, so consoles without their CPU keys CANNOT be RGH’d on that dash!)[/details]
Overall, RGH is a more convenient method, but the JTAG boot times are more stable where as RGH can take 5 minutes one boot then 2 secods the next. JTAGable Xbox’s are also much more difficult to find these days, were as RGH is considerably easier to find.
EDIT (Thanks to Tucker): Both can run multi-NAND, both can run DevKernals*
But as I stated in the beginning of this thread, BOTH CONSOLES WILL ACCOMPLISH THE SAME THING.
If anyone has any questions regarding the topic, feel free to post below and myself or another member will surely help you. If anyone has any revisions of this thread, just contact me or post below.