Vredestein makes a good tire (the Quatrac 2) in the 155/60-15 size that smart mounts on the front rims. Personally, I think that the Vredesteins are better tires than the Contis installed at the factory. Of course, you can use non US OEM sizes if you know what you're doing. smart itself sets up cars sold in non-US markets with alternate sizes.
The 175/55-15 size mounted on the rear is not as hard to find and is available from a bunch of manufacturers.
The Quatrac 2 is pretty grippy, and I think outperforms the stock Contis in hot turns and hard braking. Even if you are not into "spirited" driving, that ability to grab the road can be critical in a small vehicle where your best safety system is still the car's ability to do what you tell it to do and get out of the way of trouble. They're particularly good (for all-season tires) in cold, wet/slushy conditions. Dedicated winter treads are still the best ticket, but if you don't want to go that route, Quatrac 2 is a good choice for crappy weather where winters are moderate. But if wear longevity is your thing, the soft compound used in the Vredesteins probably won't be your best choice.