Are SOS structures better for linear phase FIR filters?
well, i see little value in factoring an FIR into quadratics and implementing them in cascade.  it won't be computationally cheaper and it's not better from a quantization noise POV if your FIR has access to an accumulator that is double width.  

perhaps it will help with limiting coefficient range so that it's less likely that you have non-zero coefficients (at the tails of the FIR) with values of, say, $10^{-8}$ the size of other coefficients (in the middle).  there are other, more efficient ways of dealing with that problem (like computing the FIR from the outside in, doing the tails first and the middle last).