which one is better for smoothing a sine wave: Low pass filters or a serially connected inductor?
You can construct a low-pass filter with a series inductor and capacitor from its output to ground. This will be a 2nd order low pass filter and if you are trying to filter out harmonics of the DDS process I'm fairly confident you'll need a higher order filter, something like an 8th order.

As the signal you want approaches nyquist, the fundamental content of that frequency reduces dramatically so you'll also need an approximation to a sinc-compensation filter. This can be achieved without too much amplitude error with one 2nd order LC low pass filter but like I said you'll need maybe another three stages of 2nd order filtering to remove the higher frequency artifacts brought about by sampling.

Depending on the frequency you wish to recover you'll get pretty decent results with sallen key op-amp filters but they have to be fast op-amps - gain-bandwidth-product of about 100MHz and these are not common or garden devices.

I'm suggesting op-amps because there are a stack of good formulas around that can help you. For instance there's a very good calculator here and produces this sort of information: -



Because the sallen-key has a low output impedance you could cut down on the number of op-amp stages by inserting inductor-capacitor filters in between but this can be a little tricky because the input of the next stage will load the output thus disturbing the frequency response.

If you go to this page (same author as the sallen key calculator you can choose all maneer of filters but the passive type you want will be under the heading RLC filters - choose the type with R and L in series feeding a capacitor down to ground.