Kits Option 1
They are great fun to build easy to follow instructions and the electronics are of good quality sound with no need to change anything. There are a few details missing off the upper and lower hulls but there are after market options you can acquire to add the detail to the tank.
If you are new to cutting and shutting and modifying your tanks then your bottom lip might drop at the thought of hacking into an expensive toy! and lets be realistic here they are not cheap for what they are.
The tracks that come with the tank standard are metal with plastic inserts more than adequate for any type of environment and if you wish to use metal tracks you will need to stiffen and support the hull and gearboxes to accommodate them and again there are options available.
Note they do not come with controllers or receivers small obstacle to overcome
RTR Option 2
Out of the box they have a few parts and components you can put on them, the electronics are very basic and can be prone to dry joint disease but 90% are fine and you won't have an issue except they all sound the same! there is a massive after market world out there for adding extra detail and upgrades available and if you want to start to cut and tuck opening hatches and modifying a tank start here! if you bugger it up slip with a knife or melt something the cost of replacement is far less than a kit.
Note by the time you have spent $$$ upgrading gearboxes, electronics, tracks, wheels etc it can end up costing the same if not more than a kit even if you buy it with metal upgrades and all you up grade is electronics i.e. ElMod
This is just my opinion and I have both options