FWIW - I had the Joyetech eGo-C as my second kit after my 901t kit couldn't cut it any longer. The batteries that come with those kits are good quality, but they are regulated to 3.2 to 3.3v so you're best vape experience will likely come from LR (low resistance) cartomizers or clearomizers. I still use the eGo-C batteries from my kit with the LR Vision CE4+ clearomizers (aka Stardust), and they work good for when I'm on the go.
My recommendation for cost a cost effective upgrade would be to get a couple of Vision CE4+ clearomizers, one each in low and standard resistance, a couple of LR cartomizers to try like the Smoktech Resurrector single coils at 1.7 ohms and the Boge 1.5 or 2.0 ohm cartomizers, and a Joyetech eGo-C Twist battery to try your hand at variable voltage. This should allow you to try different options and get an idea of what will work for you going forward.