As far as I know there arent really any bad sym and kymco models. Personally I would prefere kymco because its slightly better quality, but you also pay a little extra for that.
Like said, take a look in their brochure or visit a dealer.
Reliability is also highly dependent on how good you take care of it. If you change the oil in time both these brands will easily last 20k+ km without any problems, with a bit of luck. Theres also other parts that need replacing ofcourse like the rollers and belt but the oil is the most important
And if you want to prevent corrosion keep it out of the rain as much as possible. Cleaning and waxing also makes a huge difference. With the weather we are having now I go to the self wash every 2 weeks or so. Only takes a few minutes and 1 euro is enough to wash a scooter if youre quick