Weight alone is not a perfect measure of porosity or density. Diffusion would indicate porosity and something like specific gravity would probably be a better measure for density. I would suggest that weight is not a sure-fire way to determine if your pipe is more porous or less dense relative to a pipe with more weight.
Specific gravity is the ratio between the density of something compared to a reference material. In other words completely irrelevant to actually measuring density.
To get the density of a piece of briar weight it (let's ignore gravity, you'd be weighting the different pipes at the same gravity anyway unless you're in a plane going somewhere or the stars happen to align as you do the measurements
) and then measure the volume (by putting it in a bowl of water and measuring how much the water rose for example) and divide the two. Repeat for several pipes. What naturally follows with that formula (mass/volume) is that the lighter the pipe the lower the density.
Now that we've established that weight actually IS a sure-fire way to determine the density (no one suggested not taking volume into account - a huge pipe will obviously weight more than a small pipe - we're not stupid) the only thing left to argue about is the impact that difference in density actually has on smoking qualities.
Considering the obvious difference in smoking quality between a porcelain pipe and a clay pipe we can easily tell that porosity can have a major impact on smoking quality. The difference in porosity between two pieces of briar will naturally never reach those heights, but without some different sample pieces of briar and a lab to conduct absorption and heat conductivity experiments in it's not that easy to tell just how much the difference in density will impact smoking.