I have both a 125 shifter and a TAG kart, both are great fun but I too feel the TAG kart is a better place to start if your new to karting.
a) the TAG is about half the cost to maintain
b) I feel the TAG kart is a better tool to learn the art of chassis tuning
c) lap time wise a TAG is not much slower than a shfter
As in any motorsport, the faster you want to go is directly related to the money you are willing to spend.
Feel free to contact me with any questions you may have: [email protected]