What the online gaming community has come to know as a "JTAG" is an Xbox that utilizes the SMC exploit. This exploit causes a re-boot into the 4532 kernel. This kernel was made famous by the KK exploit (King Kong), where a bug in the hypervisor was utilized to run unsigned code. The SMC exploit is a faster version of the KK exploit (as in, it boots faster), and allows for the running of unsigned code. The SMC exploit is limited to consoles running kernels prior to the summer 09 update (7371). It is also true that there are several patched CBs, which prevent the exploit. Due to the need for a console which has not been updated, they are naturally scarce and once demanded a high price. Credits go to robinsod, tmbinc, SeventhSon, Martin_sw, and Tiros.

RGH
The Reset Glitch Hack is a new type of method which allows the running of unsigned code. Explained by free60: "We found that by sending a tiny reset pulse to the processor while it is slowed down does not reset it but instead changes the way the code runs, it seems it's very efficient at making bootloaders memcmp functions always return "no differences". memcmp is often used to check the next bootloader SHA hash against a stored one, allowing it to run if they are the same. So we can put a bootloader that would fail hash check in NAND, glitch the previous one and that bootloader will run, allowing almost any code to run." This hack requires a "glitch chip", which will send the signal. These range from the original Coolrunner-II made by Digilent, the Matrix by Infinity Mod, Team-Xecuter Coolrunner, X360Glitch Chip by SoulHaven, and the Stinger by Maximus. These chips have an Xilink chip which is capable of sending the signal fast enough for the glitch to properly function. The benefits of the RGH is that it doesn't require a non-updated dashboard to run unsigned code, meaning you could run 2 or more NANDs, which allows you to run it as a retail with one, and run unsigned code with the other. It is also able to work on the Trinity motherboard (original slim motherboard), which the SMC exploit is not. Since this hack runs on almost any Xbox (some have CB_Bs, which don't allow the hack to run, and others have unglitchable CBs: 5772 and 6752; for now), which means that the only limiting factor is the number of Xboxs produced by Microsoft. Credits go to GliGli, Tiros, cOz, Razkar, tuxuser, and Ced2911

Ultimately, since they have the same outcome, they are theoretically the same value. However, I value the Reset Glitch higher than the SMC exploit, because of the ability to still run a retail dash on Xbox Live, without consequently causing the glitch to fail.
I hoped this helped the community to understand what is the difference between the both great consoles
Kaineozxenon