Another Smart owner here - 2008 Passion - towing 4 down
The difference between towing an 1800 lb toad and a 2500-3000 lb toad will be even less noticeable. The difference in comfort and safety (and resale) will be much more noticeable with the heavier vehicle.
A common fallicy that bigger and heavier is somehow 'safer'
At 60mph a 5000# vehicle has a huge amount of inertia - when it crashes that inertia has to be absorbed by something - either the vehicle you are in or whatever you hit.
The same accident in a 1800# smart carries a lot less inertia and the extremely strong shell of the smart car is built around the roll cage principle.
They look small until you sit in one - and then you realize that for 90% of your journeys, you are carrying around a set of back seats and trunk that are serving no purpose.
They are not for everyone but we love ours, it has exceptional resale value, does 40+ mpg, can hold 70mph on the freeway and holds a weeks worth of shopping in the back :)