The 2-stroke engines are inexpensive, fast, tons of fun & there are tons of after market parts for them!
There are plenty of experienced builders here (like me) who love them & ride them all the time with very few problems, but newbies usually have a lot of trouble with them.
It really pays to NOT buy the cheapest kit you can find & do some research & spend the extra money buying from a respected vendor who will actually help you when you have problems with it.
The 49cc 4-stroke engines tend to be extremely reliable but they're more expensive, slower & don't have any after market parts to make them faster.
The most reported problems people have with them is the centrifugal clutch & the jack shaft systems... I've found the 7G belt drives to be more reliable than all the others if you know how to give them a little extra attention when assembling them. (These are what I recommend to anyone who's looking for cheap & reliable daily transportation)!
The HF 'Predator' 4-strokes are powerful, reliable, fast & fun with tons of after market parts available, but they're a lot more difficult & expensive to build properly!
That's my 2-cents.