Depends on what your system currently has.
The Transwarp will reach higher speeds, have seen stable at 15 Mhz and even one claim at 17.5 Mhz.
Disadvantage is it requires a higher power PSU and gives off a lot of heat, so heat sink and fan required.
Also overall a little more expensive to upgrade due to Transwarp parts, PSU and heatsink.
Zip GSX have seen stable up to 12.5 Mhz. Only could get mine stable at 10 Mhz, but for me good enough.
Less heat, less expense and some will say at equal speeds, the Zip is a little faster.
More than likely will work with current PSU.
It also seems that the success rate for Zip is a little higher for upgrades done by amateurs. Whereas those who know what to look for have about an equal success rate.