Here this should explain the basics of each modded console.

In general, the understanding of unsigned code is extremely skewed, and is not truly understood. Basically, most people know that if you turn the Xbox on, go to game demos, you can start XeX menu. Well after reading a post where someone said that the RGH on a console was "cheaper" than a JTAG, I got frustrated because they don't understand that they are essentially the same. If anything, the RGH is better than a JTAG because there is no re-boot necessary to run the unsigned code. But lets get down to basics. If you would like a high detail explanation of each, go to free60.org and read through the explanations.

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JTAG : Dashboard 7371 and Earlier
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 : Dashboard 14699 and Earlier
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. This hack runs on almost any Xbox, 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

RGH2 : All Dashboards
Known as Reset Glitch Hack V2. This has always been used for Slim consoles, but was updated for use with Phat consoles after the 14717 dashboard update. It runs exactly the same as the Slim hack, but does boot slower since it was not created around that Phat console. The wiring is slightly different compared to RGH1. Since the release of R-JTAG, this hack is not recommended for Phat consoles. The major contributor to this hack is Team Xecuter.

R-JTAG : Phat Consoles & Dashboard Minimum of 15572
This is an update to the JTAG exploit. This exploit gives great boot times and allows for Dual NANDs, which the original JTAG did not (due to the efuses being blown). There is very little known about how this exploit works, as Team Xecuter did not release details about the exploit. They are currently the only manufacturer of the components required for the hack. All we know is that it works fantastic and is the best choice to use for Dual NAND or an Xbox that is unable to use RGH1.

Hope this helps understand

So which is the best to buy?